Blood in poop
When my 3rd baby had food sensitivities, we did see little strings of blood in her poop. I've also seen pictures of food sensitive babes' poop, where there were little globs of blood mixed in. I'm glad you're getting into the ped. I'd take that poopy diaper to show her. Also, start keeping a food journal to see what other foods might be causing a problem (wheat gluten, corn, nuts, etc.). If it's a significant amount of blood, I'd be concerned that you're looking at a whole different problem, not related to food. Good luck today, and let us know how the appointment goes.
I think that if it is blood from the GI tract it should be a darker red. Bright red hopefully indicates constipation or a small tear in the anus, which heals up easily.
If it looks like raspberry jelly or really mucousy that is a cause for concern. Does he seem upset or cranky? I hope it is an easy fix and not something more complicated.
My first son was dairy allergic and he had bright red streaks of blood in his poop. It took 3 weeksish before it went away even after I cut out all dairy and I had to be REALLY careful. Like if something even had a trace of dairy, or was prepared on the same surface as dairy, or was in a factory that also produced dairy, the blood would come back!
I'm sorry mama because food allergies SUCK big time. I had to cut it out of my diet for 18 months...at which point I could slowly eat small bits here and there and ds was okay. (he nursed for 34 months).
Now at age 3.5 he reacts to butter, cream cheese, sour cream and yogurt, but does totally fine with hard cheeses like chedder.