Rabbi Akiva Eiger (Siman 202) wrote in the name of the Pri Chadash that one recites Borei Peri HaEtz, and if one sucked the equivalent of a K’zayit, the after-blessing is Al HaEtz (Me’ein Shalosh). Many have erred in this, understanding it to refer to sucking a grape in the literal sense, but this was not his intention. The Ben Ish Chai (also cited by the Kaf HaChayim) explained that the Pri Chadash was referring to a case where one placed the entire grape in the mouth and subsequently spat out the peel. However, if one actually sucked only the juice while the fruit remained outside of the mouth, the blessing is Shehakol Nihyah Bidvaro.