Is it a sin to drink/use to cook/use for other purposes BLESSED WATER A/O SALT?
No, it is not a sin! On the contrary. Blessed salt and bless water are meant, among other purposes, also for consumption. There is a caveat, however.
In the traditional exorcism of the salt, the priest prays: “May you be a purified salt, a means of health for those who believe, a medicine for body and soul for all who make use of you.” And it its blessing, he prays: “May all who use it find in it a remedy for body and mind.”
There are also blessings for many other foods, intended to help especially the sick, such as the blessing for wine and all manner of foods. If it is common fare, there’s probably a blessing for it.
The problem is that sometimes Holy Water is not “fresh”. The salt slows the water going bad, but it doesn’t hold it off indefinitely. I would not use anything for cooking or consumption that wasn’t just blessed and then stored well. That goes for every other kind of waters.