You might try James Blaylock and Tim Powers. Maybe both are more "quirky" than "dark", but sorta close.
Although calling them "dark" would be stretching things a bit, two of my favorite novels are modern day fantasies by Richard Knaak (semi-famous for doing some Dragonlance minotaur novel) - "King of the Grey" and "FrostWing". Also "The Janus Mask", though that's in a fantasy setting, not Earth. Darker, though.
(I think he got a book deal after the success of the Dragonlance novel, but his non-DL books didn't sell, so now he's back working on licensed fiction. But 3 out of the 4 books from that series - they all have similar covers, anyway - are great, IMHO).
And of course, if you are looking for sword & sorcery style dark fantasy, the best of that is the "Kane" series from the late Karl Edward Wagner. The books can be hard to find, but basically it's like if HPL and REH ever collaborated on writing fantasy novels. I mean, technically I guess Conan was in the backstory of the Cthulhu mythos, but never really tied in, other than a few names and common things. But the Kane stories have more overt Lovecraftian elements in them. Very dark, though. Kane is not a nice person. He's not the sort of evil that kills for fun (other than in battle), but he's completely ruthless. ( I think he's pretty much the definition of Lawful Evil, heck, maybe even the inspiration for it)