Having used Blackwood, the opponents know they have three aces and you can see the fourth. The only chance for your side to establish a trick is right now. The diamond lead may find partner with the king, establishing a diamond trick for your side before the ace of trumps is knocked out. I'm generous in awarding the "safe" heart lead three points; it is too likely to give declarer time to drive out the spade ace and set up a needed diamond discard.
Failing to lead partner's suit indicates a strong suit of your own, and a diamond lead would seem to direct the defense toward its best chance. If you do not lead a diamond, partner might not find the right shift if he decides to look elsewhere. A heart lead shows some respect for partner and might turn out right. A spade lead is imaginative, because a not-rump bid over one major usually denies great interest in the other. Nevertheless, there is danger that a spade lead will present declarer with a vital trick.
[Heart] 8 or [Heart] 5-3
[Diamond] 3 or [Club] J-1
The opponents' failure to use the Stay-man convention to locate a possible major-suit fit hints that responder probably does not have four cards in either major. This means that your partner probably does have at least four cards in spades. With a worthless hand, your best shot is to lead your shortest major. A heart lead offers similar chances. And who is to say that either a diamond or a club lead would never be right? So we give both some compensation.
The lead of a singleton is rarely attractive when it is right into the suit declarer has bid; too often it destroys partner's holding in the suit and gives declarer valuable time. Therefore, the diamond lead goes unrewarded, while the club lead, which may collect two fast tricks, gets a top mark. A trump lead may cost time as well as reduce your own ruffing power. A heart lead gets a point because it is better than the diamond opening, but it may help declarer to discard club losers on dummy's heart suit.