A lockscreen couple is what you call two people that share the same lockscreen wallpaper on their smartphone. Usually annoying overly lovey-dovey couples make a picture together and put it as their lockscreen wallpaper to show everyone their love.
Dan: "Omg, ur lockscreen is amazing, I want it."
Lucy: "Here you go! <Sends picture>"
Dan: "Omg, we're a lockscreen couple now. That's bare dank."
Stranger 1: "Gasp! We have the same lockscreen wallpaper!"
Stranger 2: "Nooo! That means we're a Lockscreen Couple!"
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."