Blues harps are most often played (in my experience) on an A harp against an "E" blues, or a D harp aganst a blues in A... guess if you're trying to match a recording though, you need to finger a keyboard, or a guitar, until you can determine the key the original is being played in and then find the cross-harp appropriate for that key. (The native key of the harp being the fourth or subdominant of the key the song is being played in).