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It was -
Black writing on aluminium (square) (1985 - 1987)
Black text on white plastic background (oval shaped) (1987 - 1992)
Blue text on greyish plastic (oval shaped) (1992 - 1999)
And the "blue label" myth is largely exaggerated. In fact, the blue label ones are probably more desirable because they have additional internal supports, drainage channels, and they don't need stabilizer bars (which makes maintenance on them much easier). Models made prior to mid 1989 have a thicker back plate, and this is really the only way that the older ones could be considered 'better'.
As Kishy pointed out, some models such as the Industrial grey or Black M13s had silver writing on a black plastic oval.