There is a great 9-page article on the MB.800 and SO.800 in Le Fana no.333, by MM. Ricco and Jung.
The story is complex, and not everything is well understood.
To sum up, there were two different Bloch 800, both with two engines. The first was the MB.800 three-seat tandem trainer, military type P3 (Perfectionnement, 3-seat), designed by Le Bihan. It flew at Mérignac in Dec. 1940. A later T3 version was considered (Travail, 3-seat), and Bloch 800B and Bloch/SNCASO 810 and 810A projects were wind tested, but no detail is known.
The other Bloch 800 was a light transport or postal aircraft, with two seats side by side. It appears under many names:
- SO.80, postal aircraft studied at Châteauroux by Le Bihan in July 1940
- SO.800, new name for wind tunnel tests in Oct. 1940
- SO.820, SO.820bis and SO.830 variants, wind tested in early 1941
- MB.800P, on the plans drawn in Jan. 1941 (P for Postal?)
- SO.800P, for wind tunnel tests in Mar. 1943
The aircraft itself was painted "Bloch 800 P N°01" on the fin. The author suggests that the confusing designations were perhaps intentional, to hide the military MB.800 to the German. Other possibilities are mentioned, such as recycling MB.800 plans for the postal aircraft.
The article also says that there was also an SO.180 project, with 6Q engines and tricycle gears, date unknown, mentioned in Les Ailes (5 May 1956).