While Paladins encompass everything Noblebright and good-hearted, Blackguards, often called antipaladins by players, serve as the spiritual opposite to their pure-spirited and benevolent counterparts. Blackguards are known to affiliate themselves with demons and serve dark, generally malevolent deities, and are described as being hated by all other races and classes that serve good. Blackguards are allowed to perform a variety of malignant actions such as using minions to do his scrimpy work for them, using sneaky and backstabbing tactics to get the better of their allies and/or enemies, or generally bully those who serve the forces of good in order to assert his overwhelmingly evil and mean-spirited dominance over others.
Non-playable Blackguards are usually found either in positions of leadership, such as leading hordes of Undead and other nasties within their posses in order to throw eggs and post dog shit through all the doors across their targeted realm, or serving as dark lieutenants for even more unpleasant and villainous entities and characters, which they know full well could end their mean-streaks by sending them to their rooms in a plane of torment and imprisonment and throwing away the key. Blackguards can also be seen as lone wanderers, either working as sell-swords due to having taken the "money is the root of all evil" philosophy in literal context or just skulking around committing crimes and making public spectacles for the sheer hell of it. To become a Blackguard, the character in question must have made a pact, or at least peaceful contact, with a summoned evil, usually an Eldritch abomination of some kind.
Blackguards in D&D
Blackguards were introduced to the Dungeons and Dragons setting through the 3.x Edition of the game, first published in 2000. Blackguards were an available class for fuckup Paladins who had grown tired of doing good and instead wanted to unleash their inner rage against all those DMs who took advantage of their good-natured alignment, being accessed via multiclassing. Becoming a Blackguard grants the player several bonuses if the character decides to trade Paladin levels.
Blackguards are automatically proficient with with most weapons and all available shields and armors (qualifying without already having these abilities is something you have to deliberately try to do however), and they emanate an evil aura that grows in power according to a Blackguard's level. Blackguards are so evil that they are capable of outright detecting those who fall under the alignment of good, primarily because wherever they go good deeds happen to be an extreme rarity, and they are incapable of accidentally poisoning themselves when applying toxins and venoms to blades. Blackguards are allowed to smite good once a day through the use of a melee attack, usually wasting this opportunity by either punching anything cute and cuddly or lashing out at their snobby, do-gooder opposites, and their aura of evil deals a -2 penalty against saving throws against all enemies within a vicinity of 10 feet.
Originally, Dungeons & Dragons 4th Edition removed blackguards; as Paladins had lost their alignment restriction and were refocused on the idea of being warrior-champions of the gods, a "blackguard" was just a paladin who was dedicated to one of the evil gods in the Dawn War pantheon. But then Essentials came along and things changed: introduced in "Player's Option: Heroes of Shadow", the Essentials Blackguard is a variant Paladin who draws power from their vices. Handled mechanically as a Striker of the combined Divine and Shadow sources, Blackguards choose a distinguishing Vice as the foundation of their powers, with the only canon Vices being Domination and Fury. However, whilst this is an easy route for an Evil character to follow, Blackguards aren't alignment-restricted; a Byronic hero or an anti-hero who accepts their vice and uses it to fuel their pursuit of a righteous cause is a perfectly acceptable blackguard in 4e.
In Dungeons & Dragons 5th Edition, character alignment got a lot less crunch assigned to it. So Paladins are no longer as vulnerable to getting their powers yanked as they were. However, they still uphold various sacred oaths to get those powers, and so the Blackguard lives on as the "Oathbreaker Paladin" class variant. He gets a bunch of "evil-themed" warlock and necromancy spells, like animate dead and hellish rebuke, control undead and dreadful aspect forms for Channel Divinity, an aura that supes up all nearby undead and fiends(friend or foe), resistance to non-magical physical damage, and a suped-up aura of heartstopping terror. An Oathbreaker can attempt to redeem themselves, but once they succeed, they're on their last chance; a 5e Paladin who breaks their Oath and falls a second time after becoming an Oathbreaker Paladin once can never become any sort of Paladin again... other than an Oathbreaker.
The December 2016 Unearthed Arcana article for Paladins gives two new Oaths, both with an "evil Paladin" motif. The Oath of Conquest is technically worded that it would also fit a Harmonium style "well intentioned extremist" (or a Hellknight-esque warlord) Paladin, but the Oath of Treachery is explicitly called out as an alternative option to the Oathbreaker for fallen or villainous paladins. This Oath has no tenets, for obvious reasons, but gives a bundle of spells related to messing with peoples' minds, or escaping from danger - charm person, invisibility, dominate person, expeditious retreat, haste, gaseous form, confusion, mirror image and passwall. They can use their Channel Divinity to create an illusory double or conjure poison on their weapons, gain an Aura of Treachery at 7th level that gives them advantage on attacks directed at victims who have allies nearby and which lets them charm an attack into hitting somebody else three times per short rest, have the Blackguard's Escape (teleport + turn invisible as a reaction to being attacked once per encounter) a 15th level, and finally pick up the ability to turn into an Icon of Deceit at 20th level. In this state, for 1 minute they are invisible (but can hurt anyone they like without dispelling it), can dictate the next action of any creature that manages to damage them, and gain a bonus to damage rolls made when they land a hit that has advantage.
Blackguards in Pathfinder
In the Pathfinder setting, Antipaladins (as Blackguards are called here) are a "variant class", which basically takes the Paladin and reverses it Bizarro-style to create a demon-worshipping, Chaotic Evil monster. this also has archetypes that fall more towards other alignments but one critical factor is clear. They are all aligned with evil. Of those other Aligned Archetypes, there is the Insinuator (from the obviously titled "agents of evil", and covers every evil alignment), and the Tyrant (who is Lawful Evil, and originates from Ultimate Intrigue).
|Dungeons & Dragons 4th Edition Classes|
|Player's Handbook 1:||Cleric - Fighter - Paladin - Ranger - Rogue - Warlock - Warlord - Wizard|
|Player's Handbook 2:||Avenger - Barbarian - Bard - Druid - Invoker - Shaman - Sorcerer - Warden|
|Player's Handbook 3:||Ardent - Battlemind - Monk - Psion - Runepriest - Seeker|
|Heroes of X:|| Blackguard - Binder - Cavalier - Elementalist - Hexblade - Hunter|
Mage - Knight - Protector - Scout - Sentinel - Skald - Slayer - Sha'ir - Thief
Vampire - Warpriest - Witch
|Settings Book:||Artificer - Bladesinger - Swordmage|
|Others:||Paragon Path - Epic Destiny|
|The Classes of Pathfinder|
|Core Classes:|| Barbarian - Bard - Cleric - Druid - Fighter - Monk |
Paladin - Ranger - Rogue - Sorcerer - Wizard
| Alchemist - Antipaladin - Cavalier |
Inquisitor - Oracle - Summoner - Witch
| Arcanist - Bloodrager - Brawler - Hunter - Investigator |
Shaman - Skald - Slayer - Swashbuckler - Warpriest
| Kineticist - Medium - Mesmerist |
Occultist - Psychic - Spiritualist
|Ultimate X:||Gunslinger - Magus - Ninja - Samurai - Shifter - Vigilante|
|Dungeons & Dragons 5th Edition Classes|
| Barbarian - Bard - Cleric - Druid - Fighter - Monk |
Paladin - Ranger - Rogue - Sorcerer - Warlock - Wizard
|Artificer - Mystic|