The Boots give you an enhancement bonus to your speed, not to something else like “your current movement rate,” or to “how far you can move this round.” As a result, your speed is enhanced, but it's still halved anyway, because blindness halves your speed regardless of what else you do to your speed.
So your character's current move speed would be (40 + 10) ÷ 2 = 25 ft.
To be clear, this isn't a matter of order of operations as thought in the question, it's a matter of wording. It depends on what the modifiers are modifying, and their exact wording. If you had an item that let you “move an additional 10 feet when activated”, then it would apply last because it's not modifying the same thing that blinded is modifying. In this case blinded just happens to apply last, but only because it halves your total speed, which includes any enhancements to it. If blinded was worded differently (“halve your base speed”, say), it wouldn't be last here.