NHL players are always looking to gain an edge over their opponents, and it looks like Andrei Vasilevskiy just found one, but this time, it's an advantage over goal reviewers.
As the rule notes, there is no rule against what Vasilevskiy is doing. While much of the equipment colour rules are mandated by the NHL so each team looks cohesive, there are no rules on blade holder colour for goaltenders. The only rule for colours is that their stick knob must be black.
With this move, Vasilevskiy is essentially making a booth review of goals nearly impossible to complete when the puck is under the skate. This happens more than you'd think too, as the puck often gets caught under a goalie's skate after going through their pads, and the backward momentum often carries it into the net.
Booth reviewers need clear evidence to overturn calls made on the ice. If Vasielvskiy's skateholders are allowed to stay black, it will be nearly impossible to differentiate between the skates and the puck.