Commentaries:
Paul is not asking what had become of the blessedness, but it is an exclamation, referring to the "congratulations" (literally) the Galatians had formerly bestowed on him. This should read, "What then was the blessing you enjoyed?"
Paul testifies that the Galatians' feeling for him was once so intense that they would have given him their own eyes! This phrase represents tremendous self-sacrifice (Matthew 5:29). In addition, given that Galatians 4:14 refers to some kind of physical infirmity, it may have been a problem with his eyes—in which case the Galatians would have gone so far as to give him their own so he could be whole again.
David C. Grabbe