In English you can be plural. When a addressing a group of people one may refer to that group as you. The word you can also be singular, used to address a single person. Your sentence is missing the context necessary for a definitive answer. It could mean either.

It's you [a group of nine people], not Tom, who are to blame. Here we have nine people in a group being addressed by the speaker as you.

or

It's you [Maria], not Tom, who is to blame. Here we have one Maria and one Tom.

Without knowing who your you is we can't say which option is correct. Outside of a quiz question, however, your use of is or are will provided contextual clues as to whether your you is singular or plural.

In English you can be plural. When a addressing a group of people one may refer to that group as you. The word you can also be singular, used to address a single person. Your sentence is missing the context necessary for a definitive answer. It could mean either.

It's you [a group of nine people], not Tom, who are to blame. Here we have nine people in a group being addressed by the speaker as you.

or

It's you [Maria], not Tom, who is to blame. Here we have one Maria and one Tom.

Without knowing who your you is we can't say which option is correct. Outside of a quiz question, however, your use of is or are will provide contextual clues as to whether your you is singular or plural.