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?
"To Blame" is an idiomatic expression.
- Deserving censure; at fault.
- Being the cause or source of something
The idiom conveys the same meaning as the expression "to be blamed"
Look at this example from the same source-
A freak storm was to blame for the power outage.
Therefore, your sentence- "He is to blame" is just a concise way of essentially saying the same thing- "He is to be blamed".