Minecraft 13w38a, Minecraft 13w38b, Minecraft 13w38c, Minecraft 13w39b
Checked on some tests, and couldn't see the custom names through the blocks as of 13w38a. However, was able to see them in previous versions.
EDIT: Persists through 13w38c
EDIT2: Persists through 13w39b, Can a mod check this out?