In the 1.9.1 prereleases, hoppers gained the ability to pull items from blocked chests. However, this behavior is functioning very inconsistently between single and double chests.

To reproduce:

1) Make a single chest and fill it with stuff.
2) Put a block above it to "block" it from opening.
3) Put a hopper below it--it will correctly pull items out.
4) Make a separate double chest and fill it with stuff. Repeat #2 and #3 with this chest--the hopper will fail to pull items out this time.
5) Repeat steps #1 through #3 with a single chest. While the hopper is pulling items, place another chest next to the first to convert it into a double chest--the hopper will continue to pull items out, even though it doesn't with a chest that was originally doubled.

I'm guessing hoppers are now supposed to be able to pull items out of blocked double-chests all the time, but the bug is that they don't if the chest was double to begin with and blocked off.