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.