I tried to make a plot markers manually as follows
PointLegend[ {Black} , {"a"},
LegendMarkers -> {Graphics[{EdgeForm[{Black, AbsoluteThickness[2]}],
FaceForm[Transparent], Rectangle[]}]}]
However, the color of resulting legend looks a bit whitened so in this case the resulting color looks grey instead of black.
Why was the color changed and how can I fix this problem?