After consulting with several blue bricks and plates from some of my older sets compared to a 2004 set, I can say that blue seems to have a high degree of varience between the years or batches. My blue baseplates are a uniform color, but many of the blue bricks and plates from sets ranging from 1998-2004 have noticeable varience. Yellow also tends to have a fair degree of varience. From this observation, I would have to say you probably just had a very noticeably varied batch, I don't think they've changed the color. Although rumor has it that the blue baseplate will disappear once the current stocks of Doc Ock's Hideout and the baseplate pack from Dacta run out. SO they could be planning a blue color change.
After a long absence, I have returned. I can't wait to start building again.