The Westermarck does not lie between Thunder and Black River. The western edge of the Westermarck is Thunder River. Only Conajohara lay between Thunder and Black, and it was taken back by the Picts.
If I may quote REH, 'No; Conajohara is lost to Aquilonia. The frontier has been pushed back. Thunder River will be the new border.' (Beyond the Black River)
I am not sure why so many maps erroneously place the Westermarck between the Thunder and Black rivers. In Wolves Beyond the Border, Howard does indicate that there is an unnamed Bossonian river on the east of the Westermarck, so it is bound by two rivers for at least part of its length.
But the Westermarck does not have the Black River as a border. Aquilonia tried that, and the Picts pushed them back.