C R A P! sent an embarrassing and mean facebook message to wrong person. how to delete it before they read it?

[Rebecca]

ok, here goes. i went on a date last week, but we were not a compatible pair...at all. in fact, he was terrible...he was not a nice person and i figured that out just by the end of the date...
anyway, a friend fb messaged me asking how it went...i gave her the cringe-worthy details, and our commentary went on ....and on... (detail i should mention: the guy i dated was still on my fb account, and i had an older message from him on there)
moving on, as my friend and i were writing back and forth in a fast speed, to keep up with one another, i accidently clicked the 'up arrow' button (which will automatically you to the next message thats in your inbox) and that next message was the guy...i didnt realize i did it until after i clicked 'send', and out of panic and cowardliness, i blocked him...but got to wondering if the message could be deleted before he reads it!? he is not a regular fb user, so i think i had the time to come on here and ask for help. if so, i suppose i'll unblock him, but he'll still notice that i unfriended him. however,id rather he see that i unfriended him, than him read what all i had said about him. like i said, he was not my type..he comes off as a huge narcissist, but i still dont want him reading what i wrote about him. i feel so bad!

[dan]

You CAN'T, but you follow it up with an apology and explanation.

[shift]

Don't want to make you panic but If he has a Facebook app or whatever on his phone or tablet he will still receive notifications with small previews about the message. If he opens it he will see the name 'Facebook user' with blank message bubbles. If he's a smart guy he will quickly come to the conclusion he has been blocked.. But by who? Obviously someone he has been in recent contact with.. He will then enter friends he has on Facebook to see who he can't find.. Your name will be among the first typed and he won't be able to find you and your messages. He will therefore realize it was you. So you're basically fucked. Well that was fun making you panic even more GL!

[Peter]

Yeah, Facebook sucks. Better get on the phone or Skype.