Blasphemy is not the problem — it refers to cursing G-d with His name ([Rambam, Melachim 9:3]( The problem is idolatry, and the [consensus]( is that Christianity is idolatry. However, the Rambam writes elsewhere (I can't find it right now) that a non-Jew only receives reward for doing the things he is commanded in if he does them because G-d commanded them; if he does them for a different reason — that they seem to be good things to do, even if G-d didn't command them — he isn't rewarded for that.