Don't buy Blitz Basic. It has since been re-released as open-source and renamed as AmiBlitz. AmiBlitz is just Blitz Basic with new features added and bugs fixed.
Also, incidentally, the versions of Blitz Basic for the PC, Mac, and Linux are a total rewrite and are completely incompatible with the original Amiga version of Blitz Basic.
If you want to write software that will work on all of those platforms and Amiga as well, stick to using C and use the SDL APIs. (Note that this will require a graphics card on your Amiga.)