You could call There Will Be Blood a black comedy. It would be a stretch, but it wouldn't be completely unwarranted. There are parts of that movie that are absolutely hilarious.
My main problem with Juno is that it doesn't really get any good until about halfway through, where it finds its heart. The beginning is full of superficial hipness, cringe-worthy dialogue ("Honest to blog!" "This is one doodle that can't be undid, homeskillet!"*), unnecessary--even amateurish--touches (the diagram of the cheerleader girl becoming a "weird girl" was entirely out of place and unneeded), and contrived plot points (fetuses have fingernails?!).** By the end, though, it manages to be happy and sweet without being overly saccharine, but the myriad problems in the first half are enough to hold it back from Oscar status. I just don't think it's on the same level as No Country for Old Men or There Will Be Blood.
*Spouts Rainn Wilson, his comic talents being entirely wasted.
**To be completely honest, the entire beginning isn't bad. Michael Cera's "I've wanted this for a long time" is one of the most subtly funny things in the entire movie. Actually, Michael Cera's character is the most consistently good part of the film.