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."