[thejeff]

A Tomb King (or any enc 0 commander) will gain 0 fatigue from fighting even wearing Black Steel (or any armor) with or without the bless.

He will get 4 (IIRC) additional fatigue from spellcasting due to the armor. Since he's casting 0 fatigue H spells this should be just 4/spell.

The 2 points of reinvig from the E4 bless will remove this and will also counter any fatigue from spells, special attacks, auras or other sources. However most such sources will build up fatigue much too fast for the reinvig to counter.

Short answer: The bless works, but he doesn't need it since he doesn't get fatigue from fighting.