By analogy with sunblock and sunscreen (assuming sunblock deflects the sun's rays and sunscreen absorbs them), a cockscreen does not block cock, she absorbs it. Thus, you are not completely blocked from the friend, but may have to sleep with the cockscreen first to get through.
"That girl Dana there is being a total cockblock for her hot friend Jennifer!"
"Nah, she's just a cockscreen. Give her some lovin' and you'll get through, trust me."