I have a sortof brand new can of ATE Super Blue brake fluid. I say sortof new, in that I opened the can a few years ago (broke the seal), poured an ounce or two into the master cylinder and then resealed the can. Knowing the propensity of brake fluid to absorb water, I am assuming it is too risky to use the can and I should pony up the $15 for a new can when I flush my fluid.

Do you agree? Or can I use the fluid since it was sealed (the can actually made a little "whoosh" when I reopened it, I suspect since it was in my cold basement but when I closed it was in a hot garage).

 

I would buy a new can for flushing.
If you were just topping off, I would use the old can.

 

Funny, I'd say the opposite.

IMO it's not so much a matter of 'risk' as quality. Is the 'old' can a bit degraded? Yes. But how long was it opened, and how much water can fit in the airspace above the fluid? If you were racing Le Mans tomorrow, I'd spring for a new can. If you're a normal human, I'd use it.
You didn't mention what size can. I generally use more than a pint when I flush.
I'm not familiar with the 'blue', but standard yellow fluid has a pretty good visual indicator of when it's old/contaminated/needs-to-be-changed.
It's like pornography (but cheaper ), you know it when you see it.