Hi, I work for a company named Rachiele, LLC where we manufacture copper sinks and I’m the patineur. You can choose to simply leave it alone and it will darken by itself over time or, if your in a bit more of a hurry, simply use a sponge (a sea sponge is best) to sponge a bit of distilled white vinegar (found in grocery stores, Walmart, etc) on it. Let that sit for a while. You can even sponge over it three or four times before rubbing it gently with another cloth and some water or another sponge and water. I call that process buttering it. Patience is going to be necessary because the patina doesn’t always happen right away. For really stubborn spots I’ll add a pinch of salt to the vinegar before I sponge it on the copper. After wiping the area with water and clean sponge (buttering it) rinse it with water and it should blend in with the rest of the sink. Hope this helps.
Q