While it seems like an extra step and in your mind it may seem unnecessary to paint an area black prior to the application of antique mirror glass tiles, here is the reason why.... Walls at the turn of the century and until the late 1970's were plaster. Unfortunately those days are gone and so are perfectly smooth walls. Because of this there are variations in the high and low spots in the wall which are normal with today's drywall but can cause issues with mirror tiles because the application call for butting them together. When applying the tiles you butt them together, if the wall is not 100% level and smooth tiny gaps can happen between tiles and if that area is white, off white, tan and so on it will be noticeable and will look terrible. By painting the wall black any tiny gaps will just look like a shadow thus giving the illusion of a PERFECT INSTALL.
Above you will notice that the wall has been painted black in preparation for the tiles to be installed.
NOTE: in the above picture not only has the wall been painted black but the installer has used a chalk line to mark horizontal and vertical application lines which are highly visible for application but not seen once installed.
Final Thoughts: Painting the wall black may be the most important thing that you can do before you install.