Late FM's were sold alongside E's. Atari wouldn't spend the cash on that.
Do you still have any copies of the Ad's? I have a ton of Pommy and US mags and can't recall seeing a blitter advertised. Not saying your wrong, it'd be an interesting to find they did.
I've never seen a STFM with a socket, space for one yes. Eary ST's didnt even have the footprint for one. Some MEGA ST's had a blitter, but not all of them. IIRC there might have been supply issues.
Technically then, getting a 520STFM is better than the 1040STF(M) because there is a higher probability of a newer MB, not the silly layout, and less RAM to desolder when upgrading...