x still have plasmid, but no Y in the beta-galactoside gene. Vectors can often self-ligate without incorporating an insert, thus yielding non-recombinant plasmid that gives rise to the blue colonies.
In rare cases, when the insert is very small and does not disrupt the reading frame, beta-galactosidase activity will occur even in the recombinant clones, so blue-white screening becomes irrelevant.