Since December 2004, GamePark decided to put new LCDs into the BLUs.
Instead of Samsung ones, they now use Taiwanese ones.
This lead to some problems, as the timing of this LCD is different.
So some programs don't run correctly on this new BLU - it is not 100% compatible to the old ones.
As the Amiga 500 once had the same problem (the rerelease Amiga 500+ wasn't 100% compatible with the Amiga 500), we decided to call the new BLUs BLU+.
The problem isn't that annoying:
New programs (like DrMD) have an LCD selector builtin, some old programs have been recompiled (like Fenix or Frodo), so that you just have to use the correct version on your GP.
There is currently being worked on on a better solution (so that all programs will run correctly on all GPs automatically).
Just don't flash your Firmware if you have a BLU+ - all current firmwares are NOT compatible!