Blackwood is awfully nice, but it's not always the best way to find a slam; in fact, it's deficient in quite a few ways. Its original intention was not to help bidders fumble their way to a slam, but rather as a method to help them stop short before slam if too many important cards are missing. Blackwood omits any information about voids, and also can leave the asker wondering just which aces have been promised.
Much more useful in a number of circumstances is control bidding*. Control bidding takes place after a trump suit has been agreed upon. After the agreement, a bid of another suit shows a control in that suit. A control is a stopper: aces and voids are first-round controls (they can win the first round of the suit); kings and singletons are second-round controls.
To make a control bid, bid the lowest suit you have a first-round control in; partner will respond with the lowest first-round control he has, and so forth. So directly bidding diamonds denies having a control in clubs, since clubs is lower than diamonds. A bid of the trump suit denies having any more controls, or just shows no interest in slam.
After you've bid your first-round controls, typically at the four level, you can continue bidding at the five level to show second-round controls if it seems like you want to go for slam.
Here's an example. You have:
A K J 8 3 2
7 5 2
A Q 10 8
Partner opens 1D, you respond 1S, partner bids 3S to agree emphatically and invite game in spades. Slam is a possibility if partner has control of hearts, so you bid 4C to show your cheapest control and initiate a control-bidding sequence. Partner bids 4H, telling you that he doesn't have a first-round diamond control (which is fine, because your void controls the suit) but he does have a heart stopper -- either the ace or a void. A return to the trump suit would have denied the relevant controls.
You can now bid 5D, showing your diamond control, reasserting your interest in slam, and denying second-round control of clubs. If partner has second-round control of hearts and thinks slam is a possibility, he'll bid 5H to show that, and you can bid 6S, knowing that you have all the suits stopped; otherwise 5S is a denial bid.
Q 10 7 6
K Q 10 9 8
A K J 8 3 2
7 5 2
A Q 10 8
What a lovely slam. If, instead of control bidding, you had bid Blackwood, and partner told you he had one ace, you wouldn't know if it was the useful ace of hearts or the useless ace of diamonds.
*Note that control bidding is also known as "cue bidding," but that's a confusing usage, since the phrase "cue bidding" has another, unrelated meaning: bidding the opponents' suit.