I don't know for sure but I tried for a very long time to mix different blocks painfully. So my experience tells that you can mix blocks only of MobiriseM4 or AMP themes. It's possible to do separately: mix blocks of M4 in only in M4 themes; mix blocks of AMP in only in AMP themes. As I understand, you can't mix M4 and AMP blocks.
I did it in a further way:
1) save block to User Blocks;
2) create a new theme and add the saved block out of User block
I did it several times but there often could be some glitches especially in AMP. I figured out that it's impossible to do with some AMP templates. It's a quite painful deal in Mobirise web page software unfortunately.