Nothing is ever quite simple. Here are a couple of paragraphs from the 2d link I posted. The Battalion was formed in 1862 but at least one company was a highland company. Note that in 1863 the black watch tartan was permitted for off duty wear along with the red hackle. These two items are the two particular markers of the Black watch regalia. In 1876 the two "Flank companies" (Grenadiers and Light companies) wore the watch Tartan, and in 1879 the entire batallion became Black Watch in all but name. The main difference was the wearing of trews rather than the kilt.
"When transferred to The Royal Light Infantry in 1863, the Highland Rifle Company of Montreal adopted the standard light infantry uniform when on parade with the unit. Only a diced band on the forage cap indicated the Highland origin of the company. Off parade, however, the company was permitted to wear trews of Black Watch tartan and a feather bonnet with red hackle.
Upon being reorganized as the 5th Battalion, Royal Fusiliers, Montreal in 1876, the regiment adopted fusilier dress: a fusilier busby with white plume, and the standard Canadian-pattern red tunic with dark-blue facings and pointed cuffs trimmed with a white crow's-foot. Both flank companies were designated as Highland, and dressed in Scottish doublets with gauntlet cuffs, trews of Black Watch tartan, and fusilier busbies. Glengarrys with diced bands were adopted as the forage cap of the Highland companies. In 1879, the entire regiment adopted this dress, and shortly thereafter changed its title to the 5th Battalion, Royal Scots Fusiliers."
While the formal regimental title didn't change until 1930, the association with the Black Watch occured in 1906. Here is another clip from my 2d link;
"The regiment's approach to full Highland status was reflected in 1904 by a further change in title to 5th Regiment, Royal Scots of Canada, Highlanders. In 1906, the regiment was designated the 5th Regiment, Royal Highlanders of Canada, and became formally associated with the Black Watch of the British Army."
As a further note; Uniforms were not supplied to the militia under the Militia Act of 1855. Privates pay was $1 per day and Captains received $2.10 per day, and the number of paid days was severly restricted. In the Montreal area the average cost of militia uniforms was $70! Serving in the Militia was an act of civic duty and pride, as well as a sacrifice.