When there is a safek regarding whether to do tevilat keilim on an item we generally don't say a beracha when tovelling the item since this beracha is d'rabbanan i.e., tovel without a beracha.
However, I have been advised to take an item that does require a beracha and simply 'use this beracha' to additionally tovel the other 'safek tevilah' items. In this way we remove ourselves from a safek.
In a case where only one item requires tevillah with a beracha, and where you also tovel other keilim that don't require tevillah with a beracha, is the beracha על טבילת כלי or על טבילת כלים? (see relevant Shulchan Aruch (Yoreh De'ah, 120: 3))
Meaning: Since only one item requires the beracha do we say כלי or do we recognise that other things are being tovelled as well and say כלים?