How do I neutralize copper sulfate blue tint on a sofa?
There are two forms of copper sulfate you may encounter, the waterfree form ($\ce{CuSO4}$) which is colourless; and the other, the pentahydrate ($\ce{CuSO4 * 5 H2O}$) which is blue/greenish (ref).  You need to get rid of both forms.

In contrast to application on organic matter, the oxidative action by bleach, regardless if based on hypochlorite or the one that is chlorine free, won't remove the stain, yet may -- depending on the dye (mixture) deployed on your carpet attack the colour. On the other hand, though, both copper salts are fairly soluble in water.

Hence, in several cyles, you might attenuate the problem:
1) Dampen the area with some of hand warm water (e.g., with a spray bottle used to dampen your clothing prior to ironing) and subsequently 2)  rapidly dry it by pressing a fresh, clean cotton tissue against the tainted area. The white tissue is meant to act like a blotting paper, to catch both the water as well as the salt dissolved in it, too.  Hence you have to move on, to expose each time a fresh, clean, and dry section of the clean tissue towards the dampened area.  

As for every cleaning of a fabric where you do not know yet how well "it works", try first with a small sector of your stained area.  If the fabric is cotton based, the polar groups in cotton actually may catch the copper salt fairly strong.

PS:  As you witnessed the accidental spill, you may be more sensibilized to see the stain, than a guest not familiar with it would be (especially as the dark fabric was not dyed uniformly).