From what I have seen at my
LFS's I found that the blue ram is slightly larger and mostly all blue. The German blue ram is smaller and considered a dwarf cichlid. Its colors range from black vertical lines around the eyes, blue and almost a yellow/orange coloring in the body and they usually have small black dots on their bodies as well.
EDIT: wanted to also mention that some of the LFS's sell the German Blue Ram as a blue ram as well, so its really hard to say if there is actually a difference. But when I went to buy some GBRs, they were not the ones that I usually see, they were almost all blue. It can be confusing for sure.....