Some Christians believe in and live according to the Noachide Laws. According to Judaism, doesn't this qualify them for heaven? I know blasphemy is in that list, but Christians Genuinely believe that Jesus is the messiah.
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.