Q: A Deacon friend was recently asked to give, and gave, a spontaneous blessing with the sign of the cross over a group at the end of a meeting. A priest in attendance stated that the deacon was in violation of can. 1169.3 and can only give the blessings allowed in the Book of Blessings and with the proper form and words. Is the Deacon wrong?
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A: This is a canonical question, which I normally prefer not to answer, but since it pertains to a blessing and to deacons whose ministry I highly regard, I consulted a local canonist for some guidance.
He notes first of all that if a priest was in attendance, normally he should give the blessing, not the deacon.
However, he cites the Exegetical Commentary on the CIC to point out that deacons have the faculty to bless persons. The only exception in the Book of Blessings is the Blessing of Missionaries (453). Furthermore, in BB 552, a leader of prayer who prays at a meeting may pray in his or her own words. The same chapter gives deacons authority to make the sign of the cross over a group gathered in a meeting (567).
So, his view is that your deacon was not wrong. But I suggest you ask a canonist in your diocese to verify this opinion for the local Church.