I have noticed an interesting thing: the Blitter in Falcon doesn't offer an interrupt capability.
Normally you can hook up an interrupt vector on MFP's IO3 port which is triggered when Blitter's BUSY = 0 again. And indeed, you can see in the STE schematics, too.
Interestingly, in Falcon the same port is wired into the DSP port's interrupt and there is no wiring to the Blitter (Combel) at all. Likewise, I don't see any Blitter interrupt pin on the Combel.
Not many documents show this, usually you can find "Blitter done" or "Reserved" description on given MFP pins (depending on the year and machines supported). And I haven't seen one single document which would describe $FFFFFA03 (and friends) with the external DSP interrupt present (bit 4).
Don't mix this with the infamous (for CT2 users) DSP IRQ line set in (very similarly looking) $FFFFA203. This allows to trigger an interrupt from the DSP when recv/send operation is possible. Also many documents are very vague about this (you are writing a vector number there, i.e. $0F -> address $0000003C, $FF -> $000003FC).