If listed properly they are different. Blue chromis are a deep blue like Alyssia said. Green Chromis are a light green color and Blue/Green Chromis are a light blue.
I have 4 green chromis and 3 blue/green chromis in my 55 and they are hard to tell apart, but if you look closely you can tell. Just letting you know the color difference isn't all that much so they are often listed as the same fish, but i was told as they get older the color becomes more aparent.