Using Blackwood with void suits does not work.. One of the solutions is exclusion Keycard Blackwood.. In exclusion Blackwood you ask partner for keycards outside of the void suit... Exclusion Keycard Blackwood (or Voidwood) is used to show a void and asks partner for key cards outside the void suit. Bids which make no sense as a cue-bid is an Exclusion Keycard Blackwood:

For example a jump over game in an unbid suit or in a suit bid by the opponents.. 

5C Exclusion Blackwood shows void in Clubs 
5D Exclusion Blackwood shows void in Diamonds 
5H Exclusion Blackwood showsvoid in Hearts 
5S Exclusion Blackwood shows void in Hearts 

Responses to Exclusion Keycard Blackwood (or Voidwood) follows the same steps as RKCB-0314: For example: Next suit means I have, 0 or 3 keycards.. Below hand is taken from 6th World Junior Bridge Team Championship:

	A K 10 7 J 3 J 9 6 5 J 3 2
	J 6 5 3 A K Q 9 7 6 4 -- A 9