The Black Watch Tartan's association with Perthshire is because the regimental HQ of the Royal Highland Regiment (The Black Watch) is located at Perth, and Perthshire was (and still is) the traditional geographic recruiting area for the regiment.

The tartan, whilst readily available today, was a "universal" tartan for the Scottish regiments of the British Army (It's official name is "Government" or sometimes "Universal"), until many of those regiments, Highland & Lowland, adopted their own distinctive tartans in the 19th century.

There is nothing wrong with wearing the Black Watch tartan with civilian dress, since you are not wearing British Military uniform and claiming to be a member of the regiment. Also, it is a fitting tribute to the Scottish regiments, which preserved so much of Scottish culture, including the kilt, after the Jacobite Rebellions in 1745-46, when the Act of Proscription forbade Scots to wear Highland dress, play bagpipes, bear arms, etc. We kilt wearers today owe a great deal to the first Scottish soldiers for keeping these things alive, since the army was exempt from the Act.

Cheers,

Todd