There are rather more Blackwater Rivers in Ireland than has been suggested above - five in Ulster alone - Antrim, Armagh, Tyrone, (London)Derry and one in Belfast. Looking at the Tyrone one on the OS map it's not that near the Tyrone Dromore - but what about Clan Owen's wild banshee? What's it doing outside Owen's country of Tir Eoghan?
As I said, what we need to know is what was in Harold Boulton's imagination - a good guide to that would be what was in his experience - where did he live? I haven't access to a biographical dictionary at the moment.