Blue Base is more common in Xbacks and HBRTG because of the Malaysian genetics where the blue base from the Xbacks originates. HBRTG is a cross between RTG and Malaysian Xback. It also can have the blue base.
Normal RTG is from Indonesia, the blue base is less common as it's not really a distinct feature, although it does happen from time to time. If the RTG was to have a really nice natural blue base and normal 4th level gold shine, it's still a RTG. If it goes to the 5th level of scales, then it would be a HBRTG. When the aros are young, it's impossible to tell them apart and breeders determine the quality of the RTGs by the parents, and assign this to the certificate based on this. HBRTG is really like a roll of the dice. You could end up with a really nice fish, or you could get a normal RTG. Xback always you get the best quality fish, and you pay premium for it.