Blue toner evens out coppery colors, so if they had any bleaching done to lighten the non-gray and homogenize their color, the blue toner would even it out.

However, grey hair that has been “opened up” with chemical treatment but has no color of its own to compliment/cover will pick up a blue tint.

Idk why it’s not common anymore; maybe because stylists know better, maybe products are better, or maybe they just use pre-formulated dyes now instead of multi-step.