Is there something grammatically strange about the expression 'To blame'?
"To Blame" is an idiomatic expression.
to blame
- Deserving censure; at fault.
- Being the cause or source of something
Source: TheFreeDictionary.com
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".