The person won't see it and you won't have to fret about how dumb it was for you to add them.
Remember that while a person is blocked, they cannot interact with
you through any Facebook channels. If you decide you'd rather wait
until the friend requests you, you should unblock them as soon as
If the person is online while you do this, they may have already
seen the request. There's nothing you can do at that point, unless you
have the power to modify memory.
Alternatively, if the person sees your request before you have a
chance to cancel it, you should still block him or her. Depending on
how facebook-savvy the person is, you should be able to convince him or
her that they only imagined the request, or that it was some odd thing
that happened by accident.
If the person receives email notifications, they will see your friend request. This is why it's important to block the person as soon as possible, before Facebook can send them an email.