One other thing, which isn't likely related to this problem, is your feeding schedule. You should be feeding him every 7-10 days, not monthly. BPs aren't like the larger constrictors that take large meals and go for weeks and months in between. I know it has "worked" for you so far, but he also may not have had any other choice. Just my two cents.

Oh, and the gooey stuff was likely urate. Snakes "poop" (even though it's more like the equivalent of our urine) when then are trying to escape something that frightens them very much.