Being a student of the 18th century (& being involved with the recreated 84th Royal Highland Emigrants: 1777-1780) 'Indigo' (or dark Navy Blue) as mentioned is the proper colour.
You can still find alot of Navy Blue balmorals & glengarrys out there.
Some commercial ones look black, but when you hold them in natural sunlight you will notice a blueish hue to them.
J. Higgins has some nicely made & priced bal's & glen's.
If your interested, woad (sort of a light or light-greyish-blue) was popular during the 17th century (another era I'm interested in & am in the process of recreating a kit for).
Hope this helps! 
[SIZE="2"][FONT="Georgia"][COLOR="DarkGreen"][B][I]T. E. ("TERRY") HOLMES[/I][/B][/COLOR][/FONT][/SIZE]
[SIZE="1"][FONT="Georgia"][COLOR="DarkGreen"][B][I]proud descendant of the McReynolds/MacRanalds of Ulster & Keppoch, Somerled & Robert the Bruce.[/SIZE]
[SIZE="1"]"Ah, here comes the Bold Highlander. No @rse in his breeks but too proud to tug his forelock..." Rob Roy (1995)[/I][/B][/COLOR][/FONT][/SIZE]
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