I generally agree with you and Iain Dow regarding the Scottish origins of the name; and also that DUFF was another variant of Dubh.
However, I suspect that "Dow" and some variants arose in some other locations totally independently of the generally recognised Scottish source/s.
For example, there is the Dow Cave in Yorkshire; and the name occurs early enough in East Anglia for it to be likely that that group is from a distictly different source.
I have also come across small groups of Dow in South Wales and the South West of England. Again these seem likely to arise from different sources; but may all be derived from the local version of Gaelic for Black.
Another guide to those possibilities is to consider the number of River Devons (Black Water) there are around the country, as also Avons.
On that theme, there is the well-known River Tyne at Newcastle; and the lesser-known River Tyne in East Lothian.