In NES version I believe it was was 1-6HP growth per level with bonus +25hp levels. Fighter had more +25 bonus levels than Black Mage for instance. If I remember right these bonus levels were guarenteed and on specific levels as well. Whether you got 1 + 25 = 26 hp or 6 + 25 = 31hp, differed tho.
In WSC and PS1, these +25 levels were moved around a bit, but I think the +1-6 chance stayed the same.
In DoS... it just seems weird....
There just seems to be no reason to it in DoS... its just random or so it seems.
In the original NES or PS1/WSC, solo black mage challenges would save before level up to make sure they got the +6hp, not anything less, because it was random, and they needed all the HP they could get.
Eh, thats just all I've heard on the subject of HP from reading gamefaqs and here and Unne's site. Not sure which one I read off of.
As far as Black Belt... his 1-50 growth was mostly changed to 1-99 growth.. so a 20 black belt on DoS equals roughly a 10 black belt on NES. However his evasion and critical rates were totally messed up I believe, but that could just be me.. he seemed to dodge a LOT more often in the WSC/PS1 versions.. which are decently close to NES.