Black diarrhea all of a sudden

I have a two year old male cat who all of a sudden last night had black diarrhea. I think it has blood in it but am not sure of the cause because he is acting completely normal (full appetite, playful, active and social). The day before he had diarrhea we ran out of his normal food (purina beyond) and had to pick up some cans from the nearby store to last the weekend. He had also started revolution flea medicine as well as gotten a second deworming shot at the vet the week before. He doesn't really eat anything he's not supposed to (non food items)but there's no way to know. I am definitely going to bring him to the vet but can't until until tonight or tomorrow morning so I'm just looking for any possible explanation or theory as to why this started up out of nowhere or else I am going to be anxious all day!

Is it tarry, or just the color black but normal otherwise. When you say you think there may be some blood in it, do you mean you can see bright red in it, or what? 

Bright red blood, if there is just a drop or two, isn't anything to worry about. 

Black, tarry stool is something to worry about...that indicates a lot of bleeding somewhere higher up .

But I'm guessing somehow this is food related, since you had to make an emergency switch in foods and since he's still acting normally.  There's not way to be 100% sure though, unless it stops as quickly as it started once that food is thru him.

mrsgreenjeens can I ask how you would define "tarry" vs very soft diarrhea that is black in color?

tarry sould be kind of sticky and shiny, I would say.

when I saw it it had dried already so it wasn't shiny but It was pretty much just a puddle so I'm assuming it was shiny when it came out since it was definitely liquidy

I would say liquidy would not be tarry.  Tarry would be sticky, and liquidy is not necessarily sticky.  I would wait and see if it happens again and then check.  You check.  You can also check pictures on-line of tarry cat stools to do a comparison.

After work I went and bought his normal food and came home and he had that for dinner. He hadn't pooped before because I didn't want to give him any more canned but his first poop after sting was totally normal. Will keep an eye on it but thanks!