Can you discard a Blend Energy card when using Rayquaza EX's "Dragon Burst" attack to do extra damage?
No, you cannot. The Blend Energy cards are not Basic Energy cards, so they cannot be discarded for this attack.
Source: Dragons Exalted FAQ (2012-08-23), TPCi Rules Team (2012-08-23)

Relevant cards

Note: All applicable cards may not be listed below.
EX3_97