Blue Diamonds are solid and uniformly blue, so not even a hint of shading. Different lineages have different bluish hues, ranging from light blue, darker blue, all the way to a blue-green tint. High quality blue diamonds are very difficult to find these days. (I know because I've been looking for them.) Ironically, their popularity 25 - 30 years ago led to huge production with little regard for quality. I saw a 2.5" BD list for $10 in a San Francisco Chinatown pet store in the late 90's, which meant that the Asian fish farm got < $3 per fish. The better BD breeders were priced out because hobbyists couldn't distinguish between high versus low quality fish.
My recommendation is that you reach out to Pingco and Pompas, both of whom are supplied by Forrest, and ask them to source high quality fish for you. If you describe exactly what you want, you'll likely get it. I also strongly recommend that you only buy from Simply Discus suppliers. Not because I have any personal biases for the SD community, but these people rely on strong recommendations by SD members for their business. Many other importers are glad to distribute to other retail channels, which insulate them from criticism of product quality. SD sponsors must delight every customer or take huge lumps in the feedback section.
If you want to know who to stay away from, PM me.
Willie