You obtain a chest
There MAY already be a legendary in that chest, say todays, but you save it.
Instead of getting that legendary your BLP assumably keeps rolling up until you actually loot it
@
psyquest people HAVE been abusing this believing they'd get more legendaries, and given all the other things being abused this expansion and blizzards lack of actual awareness as to what the legendaries current state is on live it's not hard to believe it'd go under the radar. Most people aren't willing to wait for months without a legendary to possibly get more at the end, though.
FWIW 2 months of caches by themselves is not much BLP still. You're unlikely to suddenly get 4 legendaries.