blacky, blakumen, germanic , slav

" A runic inscription from the eleventh century, found in the isle of
Gothland, recalls the murder of the Scandinavian traveller Rothfos by
'Blakumen' as he was going to the Black Sea and Constantinople. The
murder took place on the present day borderline between Galicia and
Moldavia. "

So, as we know, the lingvistic world affirm the word "black" is comming
from the german wallach where VYb in slavic mouth and with the
methatesis of "l" we got "black".
The question is, we did the Gothland people use "Blakumen"? Where
blak=slavic and "men" = germanic.
Can we assume the slavs did not got the word from the germanic people?
And if so, then what then about wethymology of "blach" then?

Best regards

a. moeller