Hello to you blues harp fans!

Today i was in a music shop to buy a harp. The shop assistant told me that a blues harp would fit best for folk and country music. He asked what key I mostly play my songs in. It is D-major. He said what I would need then is a harp in g-major. At home I practiced a little but I wasn't able to play melodies to my songs (in d-major). Did he sell the wrong kind of harp or did i do s.th. wrong?? For my it was logical to buy a d-major harp to play along with my d-major songs but he meant a g-major harp would be the right one. Please help!

If anyone knows a good online tutorial (perhaps in german) where to learn to play the blues harp pleas let me know!

Greetings, Sebastian