Let me try to get the thread back on topic now, and return to the original question about the Government sett being a "clan tartan" -- The Clan Grant Society UK has an interesting article about the Black Watch tartan here:

http://www.clangrant.org/tartan.php

The Grant Society of the US states that the BW is the hunting tartan of the Grants:

The Black Watch - Grant "Hunting"
The "Black Watch" sett is the Grant "Hunting" tartan. "Hunting" tartans supposedly developed as alternate tartan setts for the clans, with darker colors, and worn while "hunting" because it acted as a better camouflage outdoors (the belief being that the standard or "dress" tartan setts, with brighter colors, would scare off their quary).

The Black Watch tartan is the Grant "Hunting" tartan due to our clans long association with this regiment. The original Black Watch regiments enlisted men from Clan Grant, and it is said that this tartan was originally a Grant tartan before being adopted as the official tartan of the regiment. There are many clans, such as the Campbells, whose standard tartan is a variation of the Black Watch tartan (typically with lighter colors, an extra stripe, or a light stripe somewhere in the sett, etc).

-- http://www.clangrant-us.org/tartans.htm
Cheers,

Todd