You need to match the numbers in the black to make sure they're an exact match.
I don't believe I've ever seen a photo with a black background that wasn't added in Photoshop that had the same flat color throughout, so the odds of getting a perfect match all the way along all edges is practically nil.
You might want to consider removing the background in Photoshop and placing the photo in a frame with the black fill, or go with the flow a bit and use a colro that decidedly does not match so it doesn't look like you tried and failed. I'll often use a white (or some other contrasting color) line around the edge to break the two apart, too.
It's also very important that you set the prefernence for Appearance of Black to Display All Blacks Accurately or you have no idea what the really look like.
Makes sense. Thanks Peter.