From a reader…
I know of a priest who imparts blessings over the telephone. Is this valid?
The Holy Father has the authority to impart a blessing by means of electronic transmission. The faithful, who are unable to be present where the Holy Father imparts, for example, the “urbi et orbi” blessing, may receive the indulgence if they devotedly follow the rites through TV or radio while the rite is being performed.
Other than that, I think that imparting and receiving a priestly blessing requires some form of physical presence.
Of course there is the more casual sort of way of blessing someone, to invoke God’s favor on people. In that case, there is no problem at all.