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The active verb blessing indicates an event that occurs within the present; the noun blessing, then, I take to indicate a present act.  A blessing continues to give & to yield, then, like a plant that continues to grow throughout time.  My daughter's intelligence made me marvel when she was an infant; it continues to do so, so to me she is blessing (i.e., gift), in the present.

A blessing is not something that happened once; it is something that does not know how to stop happening.