These are the type of girlfriends who will deliberately try to "block" a guy from flirting with you or getting to know you better while they're hanging out with you.
For example:
You're out with your girlfriend just having fun at a bar or a club, etc.... and you spot a cute guy from across the room. After he's caught your eye and you two share eye contact for a little bit, he walks over to you and your friend...
Cute guy: "hello ladies, how you all doing tonight?"
You: "I'm doing great! How about you?"
Your "blocker" friend: (looks him up and down in a disdainful way) "We're fine"
Cute guy: "Do you all come here often?" (he's clearly smiling and looking YOUR way)
Your "blocker" friend: "No not really, in fact...we were just about to leave, come on girl"
You: (giving your friend the side-eye)
Cute guy: "Hold on a second, maybe I can get your number before you two leave" (he's obviously talking to you)
You: "Okay...sur......"
Your "blocker friend": "No, she probably wouldn't be interested. She just got out of a bad relationship. Anyway, this place is wack, come on girl let's just go"
Yea, so basically your girlfriends who are "blockers" either try to take the men who you might be interested in while you all are out, OR they try to block other guys who are interested in you from getting "too close". Sort of like a "pack mentality". Some girls do it due to jealousy or because they are attention hogs, and others do it because they're just trying to look out for you and your best interests, but they don't realize that they're really being a nuisance.