Dubh "the Vehement' King Duff I mac Máel Coluim, King of Scots - Duan Albanach

The earliest mention of Dubh and the name Macduff seems to be in Duan Albanach, a poem that dates from the time of Malcolm III. A translation is published in James Henthorn Todd (editor & transl.), The Irish Version of the Historia Britonum of Nennius (1848).

The editor says, "Duan Albanach.--The author of the following poem is unknown, but it appears from internal evidence to have been written about A. D. 1057. It is acknowledged on all hands to be of the utmost value, as the connecting link in the history of the Gaels of Ireland and Scotland .... Pinkerton calls it, 'beyond question the most ancient monument of Dalriadic history extant". pp. 270-1, n. c) Modern scholarship hasn't changed the dating, as far as I can find.

The poem is a list of kings from the legendary Albanus (brother of Briutus) down to "Maelcoluim [III] is now the king ...."

At Line 89 the poem lists, "The seven years of Dubhoda the vehement, then at Line 94 "And four to Mac Duibh".

There is some confusion over identification, but this is perhaps the earliest mention of the name Mac Duibh (Macduff).

The editor says, "Dubhoda.--This is the king who is called Cinaed, vel Dubh, in the list given above, p. 167. He is also called Duffus by some writers...." (p. 284, n. i)

He also says, "Mac Duibh, or Macduff: i.e. the son of Dubhoda, line 39 [sic].... There is evidently some confusion in these names in the Irish version of the Chronicon Pictorum, which was Lynch's authority ... but it is still probable that 'Cinaed fil Dubh' there mentioned (see p. 167, supra), was the same who is here called Mac Duibh or Macduff." (p. 284, n. k)

In other words, the translator thinks Dubh is a nickname for Kenneth II, not his brother. 

The reference to page 167 is to a different list of kings, which also ends with Malcolm III. This list is appended to Nennius. The editor says this list "appears to have come from the same source as that given by Fordun (Scotichron, iv. c. 11) ...." (p. 159, n. z)

That list gives "Cuilean, fil. Ildoib, fil. Constantini", then "Cinaed vel Dubh, fil. Mailcolaim", then "Culein", then "Cinaed, fil. Dubh". 

The translator says, "Vel Dubh.--The words vel Dubh are written over the name Cinaed by a later hand. This is evidently the same king who is called Niger, fil. Maelcolaim, in the Pictish Chronicle ...." (p. 167, n. q)

Maybe so, but this isn't the only difference between the two different lists. In the Duan Albanch, Dubh succeeds Indolph not Cuilean fil. Ildoib. In both lists he is succeeded by Cuilen / Culein, but in the Duan Albanch Cuilen's successor is Cionoath son of Maoilcholuim not Cinaed fil. Dubh.

https://books.google.com/books?id=sQ0GAAAAQAAJ&pg=PA284&lpg...