My toenail on my third toe is discolored and much darker than the rest. what would cause this?

See below: It sounds like you have injured the nail and that there is blood underneath the nail.

Fungus probably: It's likely onychomycosis, or toenail fungus. It could also be due to the nail getting traumatized. This one needs someone to see it to be sure what's going on..

Bruising: Did you hit it? If so, you caused a bruise in the nail bed, and it takes time for the new nail to grow in to replace the damaged nail.