Blasphemy is not the problem — it refers to cursing G-d with His name ([Rambam, Melachim 9:3](http://www.hebrewbooks.org/rambam.aspx?rid=14992)). The problem is idolatry, and the [consensus](http://judaism.stackexchange.com/questions/89/is-christianity-avodah-zara) 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.