I always feed my snakes F/T rats and mice.
I drop them in and once they start to squeeze, I turn everything off (ie: TV, Radio, Lights) and close the door and walk away.
I find if I stick around my BP doesn't want to eat in front of me...
I think I kind of gave him (canadianninja) the idea to let them strike and kill then walk away and let them eat in peace. (Only beauce he was having trouble a while ago getting his BP to eat)
But you need to come back in like 10-20 minutes to make sure it's all gone. Don't just assume he ate it.
My F/T rats always leave blood behind after a feed. (almost always) but it's because they have blood on there nose and when they thaw it runs.
Don't know why your snake had some blood.
When I used to feed live the snakes teeth could sometimes draw blood from the rat or mouse. That could be it.
Just scan over your snake and sheck to see if he got bit or something.
I dont think it's something that you need to worry too much about yet! Just keep an eye of them!
GL Mate!