Thanks, Hildy1. Your explanation made sense to me somehow. Comparing it to a house covering us kind of worked, however I'm still afraid of facing to some other phrasal verbs where these kinds of adjectives are used.Prepositions can drive us crazy, but in this case, as Andygc says, "in" makes sense. Think of the person as being inside the blanket, just as we are in (inside) a house. The blanket is around the person, as the house is around us.