I had the same thing happen with a batch of 1:1 nitric acid solution
when I used it to etch brass. (I was making a fancy slide clip for a
microscope--I had way too much time on my hands back then!) Anyway,
it too turned a beautiful turquoise (maybe this has something to do
with copper going into solution?), but it continues to work to this
day. I think I've had the solution for about six or seven years and
just recently used it with great success.
So, etch on!