What happens if Mew EX uses its "Versatile" ability to copy Xerneas EX's "X Blast" attack? Can it use "X Blast" again the following turn or not?
When Mew EX attacks, Versatile uses the attack name specified. So when it attacks with X Blast, Mew EX cannot use that attack twice in a row.
Source: TPCi Rules Team (2015-02-05)

Relevant cards

Note: All applicable cards may not be listed below.
EX12_88