I think you are best to wait. Food changes (especially from nursing to solids) can cause GI upset as gpm has already wisely pointed out. Obviously the vet checked for parasites and bacterial overgrowth and the kitten is not in crisis at the moment.

I would strongly recommend to stick to canned food and fresh meat etc. Grains are the evil of the pet world stay away from those cancer causing, pathetic excuse for food bits, if possible!

Yogurt with live bacteria culture, can help to keep "good bacteria" flourishing in the intestinal tract which will aid in digestion. Raw bone (take a chicken back and put it through the food processor) will naturally slow the digestion of the food giving it a chance to form in the intestinal tract.

Fresh looking blood in the stool is most likely irritation of an already sore rectum & anus. This type of bleeding is very slow to heal as everytime kitty takes a dump, it has to go past the site of irritation. Eventually it will heal as the GI straightens itself out.