Now when I took out the forst card and tried the Blizzard, one thing that scared me is when I popped the Blizzard in, I got some weird lines on the screen (akin those that flash sometimes on loading screens), and then the infamous software error...
I booted with both buttons, screen came up fine, booted into a floppy with sysinfo and all looked good too.
From memory booting with both buttons will have disabled the card.
The ram jumper may be set incorrectly
http://amiga.resource.cx/manual/Blizzard1230_jumpers.pdfTo start with you may want to set the jumpers to 0mb ram and kickstart in rom. I'm not sure if fpu clock will make a difference in booting, but setting it to synchronous might be worth doing.
If it still doesn't work then try removing the oscillator/fpu/ram, if that still doesn't work then there may be a problem with the card or your a1200.