Which radio frequency is used during a flight interception?
An interception with an aircraft that's either on an unknown frequency, or non-responsive etc will be conducted on the emergency frequency of 121.5. Likewise, a pilot would be expected to tune into that frequency if they found themselves the subject of an interception.

There is no other standard "air to air" frequency that's likely to be utilised. I'm aware that some countries have special frequencies reserved for formation flight etc, but most pilots wouldn't have those memorised and wouldn't be expected to use them.

However, if the aircraft is/was communicating on a known frequency then there'd be no reason that an interceptor couldn't use that.