You popped a tick w/your nail?
I see one of those things & I freak out, can't touch it...maybe it was a flea? Ticks are kinda on the big side, esp if it's been engorged w/blood, they get real fat! If it was a tick, your guy needs to be seen by a vet, esp if that tick was engorged, they can pass tick borne diseases to your dog.