Itasan, do you want contemporary English equivalents (potentially, to the Japanese meaning), or old-fashioned/dated/archaic ones? (Good though on tigertiger for finding a word that resembles the Japanese one in at least form!).
I can't recall ever hearing or even reading 'blackguard', and the synonyms that my thesauruses are suggesting can all be used playfully (in fact, I suspect that is their primary use now)...so, if this 'black stomach' in Japanese is used to describe a TRULY evil person, not just a bit of a moral "reprobate", I'd suggest you still keep looking for alternatives. It would help if you could tell us if any (infamous? Contemporary?) Japanese people have ever been called 'black stomach' (what's that in romaji, and kanji? Kuroi...hara? Onaka?) or similar, and why (that is, if it is a general term of abuse, what earns it? Is it offensive, or playful, or just plain strange now and dated?).
Micheal Quinion gives an account of the word's history:
http://worldwidewords.org/weirdwords/ww-bla3.htm
The OALD7 has an entry and a concise yet informative account of the origin of the word (at least on the CD-ROM version); the CALD1 also has an entry; the LDOCE4's entry and origin are both pehaps a bit too concise; and the Collins COBUILD on CD-ROM (incl. 3rd edition of dictionary) has no dictionary entry but a range of synonyms in its thesaurus section. (Unfortunately my New Oxford Dictionary and Thesaurus of English's CD-ROM doesn't seem to function on my new PC here).
Oxford: Ñ 'bl&gA;d; NAmE Ñ -gA;rd/ noun
(old-fashioned, BrE) a man who is dishonest and has no sense of what is right and what is wrong
Cambridge: noun [C] OLD-FASHIONED a person, usually a man, who is not honourable and has no moral principles
Longman: [countable] old use a man who treats other people very badly
COBUILD: noun scoundrel, *beep* (offensive), bounder (old-fashioned Brit. slang), rascal, rogue, swine, villain
BTW, from "reprobate", I came across some interesting phrases on the Cambridge CD-ROM's SMART thesaurus:
the lowest of the low
a nasty piece of work
the scum of the earth
a wolf in sheep's clothing
I have no idea if they are the equivalents you're looking for, but they are generally more forceful than e.g. the COBUILD synonyms.