The ordinary Grant Hunting tartan is indeed the same as the Black Watch. However, there is another Grant Hunting tartan that does resemble the MacLaren.

This latter tartan is first recorded in Wilsons of Bannockburn 1819 Key Pattern Book. It is very similar to another Wilsons design, called Regent, which was a popular fashion tartan in its hey day. It was not uncommon for Wilsons to produce several tartans along the same theme under different names.

Well, after George IV ascended to the throne in 1820, the Regency was over, and the name of the popular "Regent" tartan became outdated. So Wilsons gave it a different name -- MacLaren. According to the STA notes on this tartan, it was adopted by the MacLarens c. 1822.

So there ya go!
M