Today is the Feast (Solemnity, really) of the apostles Peter and Paul. This ends the Pauline Year, and leaves us completely to focus on the Year for Priests.
The Handbook of Indulgences lists this:
The Christian faithful obtain a partial indulgence when they make devout use of a devotional object (such as a cricifix or cross, a rosary, a scapular, or a medal) which has been rightly blessed by any priest or deacon.There is a footnote that says that to be rightly blessed, a proper formula ought be used from the Book of Blessings, but notes that a sign of the cross, even without words, is sufficient, assuming the intention to bless was present.
If the devotional object has been blessed by the Pope or by any bishop, the Christian faithful can obtain a plenary indulgence while making devout use of it on the solemnity of the holy apostles, Peter and Paul, provided they add to its use a profession of faith made by any legitimate formula. (EI 35)
The normal requirements for indulgences are also, of course, in play.
As an aside, the only object I have that I know was blessed by a bishop is an old and broken scapular, but it still holds sentimental value, and can still be devoutly used, just not in the same way as before.