I have two iron helmets. One with Protection III, and one with Unbreaking II and Blast Protection II. If I try to combine them on an Anvil, depending on the order, my options are either Protection II and Unbreaking II, for 8 levels, or the same enchants for only 2 levels. It will not let me add blast Protection II to the Protection III and Unbreaking II.
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Yes, but the the Protection II should go away in favor of Blast Protection III. At least when placing the item with the Protection II in the sacrifice slot, the Blast Protection III already on the main item should be kept, and add the Unbreaking II from the sacrifice.

Cannot confirm. The enchantments of the sacrifice slot are added if possible (like Unbreaking) but not added if combination is invalid (Protection / Blast protection)
Yes, that is how it should work, but not how it is working: the conflicting enchantment on the sacrifice item is being added, and replaces the better enchant that is already on the main item. When there is a conflict, the one that is already on the item should remain unchanged.
Oh nevermind... I must be mixing myself up. Too much minecraft lately on too many worlds...
Blast Protection is a subset of Protection, therefor you cannot have both.