Yes, there really is a Loch Lomond whisky
, but the fact is that Hergé didn't know that at the time he inserted it into the books (for the 1966 version of The Black Island
The intention had been to remove references to Johnnie Walker, another real-life brand
; it was felt better to fictionalize the name so that there was no need to ask for permission to use it, and I imagine so that it lessened the chance of some young copy-cat getting drunk by accident when playing at being Captain Haddock.
Loch Lomond is well known as a Scottish beauty spot, which has long been immortalized in the popular song of the same name, so I imagine that it was seen as a very good choice - it sounds right. There may even have been attempts to check that there wasn't already a whisky bearing the name - and they were probably delighted to find that there wasn't, so that's what they used.
It turned out that there was
a brand with the name - it just wasn't in production at that time, so the fact must have slipped past the Studios.
Then the whisky went back into production, and things came full circle.
So the answer is, Hergé didn't base his whisky on anything other than imagination - but accidentally came up with someone else's idea!