Is there something unusual about the phrase: 'He is to blame'? It seems to be a shorter form of the passive 'He is to be blamed'. Does this make it some kind of adjective with a to-infinitive form? Are there any other phrases that follow this pattern?
The idiom conveys the same meaning as the expression "to be blamed"
Look at this example from the same source-
Therefore, your sentence- "He is to blame" is just a concise way of essentially saying the same thing- "He is to be blamed".