Actually, I think you are wrong. I mean, it depends on what you call a local bakery and what exactly you would define as black bread (I use a very loose definition), but to my knowledge most bakeries offer some kind of black bread and every super market has it (often not fresh).

Sure, the assortment of wheat based breads is larger and very small shops might not have it at all, but overall it is pretty common (the two bakeries within a 100m radius of my place both have it). However, if you are looking for top quality black bread you might be out of luck with your closest bakery, but should find a shop within a few kilometers. Just to give one example:

'Bäckerei Junge' is one of the common bakeries: https://www.jb.de/geschaeftefinder/

And one of their high quality black breads is the 'Kerne & Saaten': https://www.jb.de/produktfinder/aus/hamburg/in/brot/produkt/...