My cat had this. I gave him Katz n Flodder (sp?) dry food. I also made plain
baked chicken with enriched wild rice mixed 50/50. Chop up chicken with skin
and fat into bite sized pieces and mix with rice. Store in small containers in
freezer and defrost as needed.  You might also want to mix some vitamin/mineral
powder into it. It'll take two days for her stools to get better. Don't let him
eat anything else, not even a bite or he could end up with the bloody mucousy
diarrhea for three days. My cat had the extreme type.

You might try Science Diet Sensitive Stomach.  It limited Gideon's
irritable bowel flare-ups to about once a year or less.  But since
I had to switch to canned when the other went diabetic, he's also
doing very well on Wellness canned.

Or, you might see if she'll eat some canned pumpkin.  Visible blood,
rather than black stool, means it's fairly close to the rectum.
Some soluble fiber like the pumpkin could help soften stools and help
them pass easier.  Some cats love pumpkin, and others turn their nose
up at it.  You can blend it with canned food, too.

Your vet should have low residue perscription food in his clinic.  Comes
canned or dry.  We use it sometimes to help bring diarrhea under control
with kittens (though there is no kitten formula of low-residue).

Sharon Talbert
Campus Cats

Didn't your vet prescribe you a food based on your cat's condition?  What you
are looking for can only be purchased through a veternarian.  I would think
about switching to a veternarian that wants to treat the condition, not just
diagnose it.

Prescription Diet Feline i/d (or Feline w/d) might be a couple of options.