Blackened portions are not good and often are a sign of a bacterial blight, Psuedomonas syringae. Not sure you will be able to tell for sure until spring and the tree starts to leaf out. Repost then.

btw - best to put this in the Maples forum at that time. This forum is extremely inactive and is not really intended to be plant focused - JM's are used in many settings that have nothing to do with a Japanese garden.

Thanks!