Are you leaching copper? Can you give any operating data?

It might be Prussian Blue created by incomplete pyrite oxidation to Fe+2 which in turn reacts w/CN to form the intense blue ferri-ferro cyanide complex...the color may persist & CN consumption will be high until you are free of sulfides in your feed...access the following paper for more info: 

Dissolution behavior of iron cyanide (Prussian blue) in contaminated soils

I think, you should provide more information. It is important to know if the color of the pregnant solution was similar or different in the previous treatment campaigns. Apparently, your vat leaching plant treats oxide copper ores, try to review the copper feed grade. If the vat leaching process received a high grade material, the solution color would be dark blue. For that reason you should review the historical information.

Some copper leaching operations prepare pregnant solutions of different copper content. The idea is to have a preliminary idea on the copper dissolution and the mill feed grade. This is a comparative analysis based on the solution color. 

Other point to keep in mind is the sulphuric acid consumption. If the consumption is higher and the gangue minerals are similar to the previous campaigns, and the copper grade is higher, you will need more acid.  Copper dissolved is the actual (or expected) amount of Cu dissolved during the vat leaching process based on the leach performance data. It takes into consideration variations in head grade and would allow for significant reduction of the leach cycle for the current head grades.

this is an Fe-S-Au ore.

This deep blue was coming from my tank with a heavy blue foam weakening the cynide.