The releases in the branches (eg 1.4 branch) don't make any drastic changes to anything, they are for bug fixing only. So yes, it uses the same blocks as always.
The trunk (ie 1.5 dev) is development code for developers to work together and make larger changes. In the case of blocks, only some 3rd-party blocks will break in the new block structuring (all the standard blocks were updated at the same time). Unfortunately this was necessary to get a much more flexible block structure - hopefully it won't happen again.