I believe I found the perfect National Geographic article for you.
- Blood oxygen carrier - element - color - example animal
- Hemoglobin - iron - red - humans
- Hemocyanin - copper - blue - many octopodes
- None - none - clear - ocellated icefish
- Hemoglobin (which breaks down into biliverdin) - iron - green - green-blooded skink
Special: Hemargentum - silver - brownish black - I made it up
The purpose of these "carrier" materials is to make oxygen transportation more efficient. The ocellated icefish does not need a carrier material because it lives in water that is highly oxygenated already. Carrying this over to silver, the obvious transport compound would be silver oxide. Unfortunately, I believe silver oxide is chemically difficult to make, and this would result in a lot of wasted energy.
There's also the possibility of silver sulfate (Ag2SO4). Unfortunately, the standard way to create silver sulfate requires silver nitrate (AgNO3), and if you already have silver nitrate, I can't imagine turning it into silver sulfate is going to buy you more efficient energy transfer.
The problem I keep running into is that figuring out a plausible reason for something to have silver-based blood requires some deep organic chemistry knowledge. I hope I've given you a start, but you might need to find some biochemistry boards to ask on instead.