I did once see a Test in black whipping with the later 'two fishes' logo, rather than the 'happy angler' gold oval, so I'd always presumed that black silk denoted a later rod, just as it does with Edgar Sealey rods.
All the other Tests I've seen have been done in what I took to be yellow and black twist, often called 'jasper' ( A Southern nickname for a wasp). However I now believe it was black and
white nylon with the old-fashioned oil-based varnish giving the white that 'yellow' tint, just as was the case with Aspindale rods.
As to when Tests were first made...well, I've never seen one with brass furniture, so I'd say post-War.