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 hotfriend Jennifer!"
"Nah, she's just a cockscreen. Give her some lovin' and you'll get through, trust me."
Nickname given to bales of marijuana thrown overboard or out of airplanes. Origin is reportedly from members of the US Coast Guard in the area of Southern Florida.