Bright red blood in the stool is scary, but usually isn't serious if you only see it once or twice. It can just mean she has a tummy upset (acorns would do it!).
Black tarry stools indicate blood is actually being digested which is much more serious.
If she continues to have a bloody stool, though, it could be HGE. Yorkies are prone to it and it is very serious. It can be fatal if not treated quickly. Here is some information so you know what to watch out for:
Hemorrhagic Gastroenteritis (HGE)