How big is the spot? I occasionally get a dark red spot the size of a pinhead under my eyes. I think it's a broken blood vessel, and I usually only get them when I stay up longer than twenty-four hours straight, especially if I spend a long time on the computer. The spots usually go away in a few days though. Maybe it could be something like that? If it's the size of a bruise though, I'm not sure what it could be.
As for concealing it, I imagine a yellow-toned concealer, under your foundation or normal concealer, would work well since they're used for under-eye circles which tend to be a purplish colour. If it doesn't go away in a day or two and you're concerned about it, you should probably see a dermatologist.