Sometimes on these 'blank on both side' discs there will be a small lcode number/reference number stamped near the centre hole: this side is the 'top' i.e. you write/burn to the other side...
Thanks for all your answers, the only difference between the two sides of the discs, is that the side which is normally printed on has a slight bluish twinge when you hold it to the light.
Chuck, your suggestion that they may be plain so that they can be printed on, seems very logical to me.
If they are printable, there is a thermal dull-looking layer.
Un-necessarily expensive for normal use.
The bond of thin film labels (which are the best) is better with silver.
Tinged surface is the burning side.
Al.