Opening leads against various doubles in competition cause us many headaches! Double trouble for sure, what do they all mean!
If a suit bid is artificial, a double shows that suit, 5-cards or longer with a minimum two of the top 4 honors!
e.g. over Stayman: 1NT-p-2♣-X- (artificial) double by opponent.
However, when Blackwood is used at a high level a double at the 5-level is LEAD DIRECTING, asks partner to lead that suit. You may have the Kxx or KQx and an outside Ace. Before they get to set up their long suit, we double asking for that suit led while you control their long suit for discards.