North's four no trump was not Blackwood; it was just a mild, if overoptimistic, slam suggestion, which South was delighted to accept.
I am sure that you and I would have led a spade even without the aid of East's spade overcall and even if for the wrong reason: there is more chance to win defensive tricks in a short suit than a long one. However, West led a club, and declarer made a somewhat shamefaced 12 tricks when he drew trumps and shed three of his four losing spades on dummy's good hearts.
The final score of the U.S.-U.A.R. match was 52-35. The swing on this one deal was 1,570 points—worth the entire 17 International Match Point margin of victory. I leave it to you to decide how much of the swing should be credited to East's overcall.
Remember where I said that three no trump was North-South's only makable contract? This is not entirely true. Bizarre as it seems, if North-South could find a way to reach four spades, that game cannot be beaten!