Blue was a common colour for dying of wool - it would be indigo, presumably.

There are many blue based Scottish tartans and there were the blue bonnets they wore even when the tartans were forbidden. Some scraps of old fabric which have a deal of white in them when analysed forensically were originally mainly blue but it has washed or bleached out over many years.

Of course the Scottish saltire flag is mainly blue, so just the word 'blue' could infer Scotland or Scottish.

However - I was once asked about blue heather - the bonny blue ling heather...