John Blanche is a British fantasy and science fiction illustrator for Games Workshop. Blanche originally became associated with Games Workshop in 1977, supplying the cover for issue 4 of their gaming publication White Dwarf, and producing the cover for the first British edition of Dungeons & Dragons. Apparently he's fallen ill in recent years, though is still active in creating model concept sketches for AoS and 40k.
Surprisingly, John Blanche was perhaps once the single most incredibly skub thing to come out of Games Workshop, prior to Age of Sigmar. It usually seems there is no middle ground on his work; you either think he single-handedly set the art style for the golden age of Warhammer Fantasy and created the atmosphere of Warhammer 40,000, or you think he's a (rather intelligent) monkey with a penchant for red crayons.
Blanche's work is similar to that of Ian Miller, although while Blanche's style is focused on the feel of an image and an over-the-top "metal" look with lots of blended (red) colors creating a sketchy look, Miller's is focused on massive amounts of detail and sharp lines. Both are very similar in composition.
For anybody interested in his drawing process, here's a video of him doing some Age of Sigmar art and here's a video of him doing some Warhammer 40,000 art.
Blanche's illustrations are marked by heavy texturing to portray a used, gritty world. Other typical features are exaggerated proportions, skulls, and intricate detailing (especially implants), which all take a fair amount of skill to produce. He also has a fetishe for the colour brown. Given that he admits Rembrandt is an influence on him, this is probably a reason why he likes the colour - another, recently revealed on the AoS podcast, is that he dislikes the way blue paints tend to fade over the years-long process between creating a concept sketch and producing a finished model. He is largely responsible for most of the tone of the grim, dark, grimdark future; although the universe was inspired by many sources, and many authors and artists contributed to its development, none capture it quite as well as Blanche does. Many of the coolest aspects of the universe were inspired by or directly taken from his artwork.
Blanche's illustrations are marked by heavy texturing to portray a used, gritty world. Unfortunately, this means it's usually difficult to figure out what in the hell you're actually looking at. Other typical features are exaggerated proportions, skulls, and "intricate" detailing (although a similar effect is found on many kindergarteners' scribbles), all of which look absolutely ridiculous. He also gives every faction weird beige bio-mechanical tentacles to stand next to. Many on the pro-Blanche side believe he "set the tone" for the universe, ignoring the contribution of the dozens (even hundreds) of other sources and contributors. Other artists capture the universe in much higher detail. Many of the coolest aspects of the universe are completely unrelated to Blanche, and in fact would be much less cool if he had been involved. Blanche is also sadly obsessed with the color red, and is believed to steal red crayons from unsuspecting children.