Difference between unit and target unit?
The difference is actually pretty similar to the difference in Magic: The Gathering.  When the word Target is present, the effect must point directly at the unit, and will trigger abilities that look for the unit to be targeted. Blood transfer, however, is worded this way to prevent abuses of similiar effects.

Assume there's a unit that you control that says 'when this card is targeted, do x".  Blood transfer would let you do X on demand, which could cause things to spiral out of control.