As far as I understand, there is only a difference with regards to the context and subject and whether the phrase is being used for an indirect or direct object, which has been mentioned before. If something is bad, boring, stupid or no longer fun then "it blows" or "it sucks". Change the object/subject you are referring to and the meaning changes. If you're talking about how good the vacuum works, "it sucks" up the dirt really well. If you're talking about the wind - it blows or it blew the clothes off the washing line. You do not necessarily need to have the object or the situation at hand however to use that phrase. You could be talking about a class you attended earlier and say - it sucks. There could be a situation between you and a friend and - it sucks. Or you could be talking during dinner, to friends/family, about how hard you worked on your studies only to get a grade that wasn't as good and - it blows.