Grimdark blogs are NSFW blogs that feature mature content along the lines of excessive violence or course language. Although there are degrees in grimdark blogs, one should not be surprised to find murder and/or rape among the subject matter of such blogs.
Grimdark blogs often overlap, more so than other types of pony blogs. Ask Discord Whooves author Jitter made this graph of the relations between several grimdark blogs. See original post for more explanation.