Even though i leveled out of blitz a while back, when i had to use my BB1 in GB2 i can vouche that It. Is. Terrible.
I like the idea of having raised to BB2, but then at the asme time think about the DD,CL that while have a hard time competing with BB1s already, will now have to deal with BB2's.
On the other hand, remember blitzkrieg is basically to get you used to gameplay style, and how to properly control your ship/guns. Introducing BB2s to a game mode where new players have just started, would probably end up making them not wanting to play because now Blitz, and GB are too hard for them. Just saying from you and my point of view it would be easier and fun, but we also have to account for the new players that will have to cope with Ships that they would not even stand a chance against.
Imagine a BB2 Fuso in Blitz. 6 R slots. Each with dual 14'' that a volley of 12. 14'' rounds per 10 seconds really.