Hi Art,
now you got me 
The blue dials used to be available for WG models and in fact the 1815 Up&Down ref. 221.027 does not have a black dial.
But it´s hard to tell, so it seemed to fit 
Personally i found the dark blue dials to be quite stunning and an interesting alternative!
Frankly, i have NO idea how long these used to be available (but guess that´s been available all over the production time of the 1815 range).
Cheers,
Peter