When you click on either key, what level access does it list (or is shown on the enter BF icon)? There are 2 level 1 BF keys in act 1, so if you found the one in the starting cell block in the locked chest, the second one will not open another dungeon.
If that isn't it, are you sure you are checking the second dungeon, and didn't bypass it and find the third dungeon?
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