Next time, maybe you could link the game *and* tell us to look at turn 24? I spend ages wondering what was so special about that blockade in turn 5, or the one in turn 15..., 21 then maybe... :s
Isn't that pretty standard...? You block a choke point, to prevent a huge amount of troops to rush into "your" continent. Sure, you'd prefer doing it on your opponent's side (in this case, Spain), but it's better to sacrifice North Africa than all of your Africa bonuses.
I really liked the turn 24 one though, which at first made me think "did he click the wrong territory!?", but then on second thought started making a lot of sense.
You actually didn't lose a bonus (you didn't have Egypt anyway and it would've been tough to conquer it), even though it kinda looks like it at first glance.
Secondly, you're rather unorthodoxly not blockading one of your front-line territories (your opponent never even saw the blockade happening!), but it really was the perfect place for it. First, because he didn't see it, you allow your opponent to waste armies and a turn on attempting to capture a bonus which now has a wasteland in it. Secondly, because of Congo also being a wasteland (and Congo and Somalia together blocking off the way to South Africa), you *are* saving that bonus.
So, for non-standard, but very clever use of a blockade card, my vote is on the turn 24 one.