2016
http://www.booths.co.uk/
Although the term is now commonplace - and more than one tea supplier produces a blend by this name - we can find no written reference to 'Builder's Tea' before this in the 'Independent' on 6 Feb 1996; "We had all taken to bringing our own builders' tea bags down to breakfast, as the tea provided was a perfumed herbal concoction." (OED). The first literary reference we can find is as recent as the following in Rosamunde Pilcher's 'Winter Solstice' of 2000;
‘You like builders' tea, don't you?’ ‘Strong and black.’ She poured her own mug and left the pot to stew.
It is prticularly well described in Neil Gaiman's 2013 novel 'The Ocean at the End of the Lane';
I drank some more tea. It was still hot, and strong enough: a perfect cup of builder's tea. You could stand a spoon straight up in it, as my father always said of a cup of tea of which he approved.