When it comes to pairing a blazer with black trousers and a black shirt, there are a few color options that would work well. Here are some suggestions:

1. White blazer – A white blazer would create a high contrast and sharp look, making the black shirt and trousers stand out.
2. Grey blazer – A grey blazer would create a more subtle contrast and provide a neutral background that would complement the black shirt and trousers.
3. Navy blazer – A navy blazer would add a touch of color while still maintaining a classic and sophisticated look.
4. Burgundy blazer – A burgundy blazer would provide a bold and unique look that would make the black shirt and trousers stand out.