Is boiling fish in tea to change its color to black—because it indicates its quality—considered a betrayal, and would the embarrassment be removed by informing the customer of that?

If what is done to the fish is considered a defect or a decrease in value by customers, and the buyer is unaware of it and it is not disclosed to him, then it is considered prohibited deception (ghish). However, if it does not affect the price or is not considered a defect by the buyer, or if the buyer knows about it and accepts it, then there is no harm in it and it is not considered deception.