If you have a suit in which your side could be off the first two tricks (such as Qxx, or Jx), you shouldn't use Blackwood. For example, Opener holds:
AKJ7642 
AKQ 
32 
3.
You open 1 and partner makes a limit raise to 3. You expect to be in the slam zone. This is not a Blackwood hand. Let's suppose you use Blackwood and partner shows 1 ace. Now what? If you jump to 6, you could be facing:
Q1093 J32 Q54 AQ5.
You are in a slam off the ace-king of diamonds--no good. But, what if partner's "one-ace" hand were:
Q1093 Q32 AK64 42?
Now, slam is laydown. So, Blackwood should not be used when you have a suit off two quick losers. The solution for these non-Blackwood hands lies in cue-bidding.
“Cue”-Bidding
First, let's explain what a cue-bid is. The term "cue-bid" is a bit misleading. If the opponents open 1 and you bid 2, that is a Michaels “Cue-bid,” showing spades and a minor. This has nothing to do with a "cue-bid" for slam bidding. I prefer to use the modern term, "control-bid" (more specific than the term "cue-bid"). When looking for a slam, we show a control. A "control" is an ace or a king (or, if in a suit contract, a void or a singleton). Think of a control this way: "If we have a control, the opponents cannot take the first 2 tricks in that suit." We use control-bids when we have agreed on a trump suit and are moving towards slam.
Example:
AKJ7642 AKQ 32 3.
You open 1 and partner bids 3 (invitational). We saw above, that Blackwood is useless. Instead, you "control-bid" 4, to show that you are interested in slam, and have a club control. In this case, your "control" is a singleton. You know that the opponents cannot cash 2 club tricks. You are worried that they may be able to take the ace-king of diamonds. Your partner will now control-bid 4 for you, if he has a control in that suit. If he has:
Q1093 J32 Q54 AQ5,
he hears your 4 control-bid, and will have nothing to contribute. He will bid 4--he has no control in either red suit. Opener will pass 4.
What if instead, responder held:
Q1093 Q32 AK64 42?
He would control-bid 4 and opener would know there are no suits off the first two tricks. Opener could then use Blackwood to make sure two aces weren't missing.
When is a new suit a control-bid?
A good rule of thumb is:
Below 3-of-your-major, there are no control-bids. Above 3-of-your-major, once a fit has been found (a suit has been agreed), a new suit is a control-bid (ace, king, void, or singleton).
Example of NOT a control-bid:
1 2
3*
*Not a control-bid. This says nothing about the ace or king (nor length) in clubs. Most partnerships use it as some sort of "naturalish" game try. It is not a slam try.
Example of a control-bid:
1 3
4*
This shows a control in clubs and slam interest (don’t ever control bid without slam interest).
Bid controls in order (whether first or second round). This is the so-called "Italian" method, which I highly recommend. Don't be concerned that you would show a singleton or king before an ace or void--up the line (in order) is the key to making this work!
The trump suit is not a suit in which you make a control bid (RKC will eventually be used to make sure you aren't missing the ace-king of trumps).
Never jump into a control bid.