These things have invaded our house. I killed 50 of them last night, and there are more back tonight. They're about 3mm (1/8 inch) long. They pretty much just sit up on the ceiling unless disturbed.
When I kill one, it leaves a little bloody smear, like a mosquito. So I'm pretty sure they're feeding on something living, as opposed to being fruit flies. But we're not all itching, so maybe it's our dog or the guinea pig?
To the naked eye, they appear pretty much solid black, and only when you look at the body really closely do you see muddy yellow stripes.