Blatt will be fine, and so will the Cavs.
Blatt has experienced rough starts with new teams almost every year that he has coached, and almost every time it ended up with a title.
The aspect of his coaching that Blatt is most lauded for is his adaptability to his players, but this comes with a price. It takes time for him to learn his players, for them to learn him, and for a system to be constructed that maximizes the players' talents. But when it does happen, opposing teams rarely have answers to what Blatt throws at them.
Keep in mind that the Cavs are playing in an extremely week East, so they can afford to suck for extended periods of time and will still likely be a top-3 seed. I expect the Cavs to have lots of ups and downs for a while. I think they will really start clicking after the All Star break, which is around the time of year where Blatt's teams usually rise from the dead and start eating alive every team that's in their way.