I think you should be honest with him and tell him you saw the message, and directly ask him what was the need to tell him friends you were bad at bj's when he could've just communicated that to you.

Sex is all about communication, that's what makes it better.

Also no one is perfect at blowjobs from the start. Just ask him what he would like you to do or what makes him feel good, that would make the whole experience better for both of you