When I set a black_box attribute to False or No in the VHDL RTL, it is always taken as True or Yes by Vivado Synthesis.
In the synthesized netlist, "am_err_cntr" is a black box despite what I have set.
The black_box attribute is used to make something a black box. The default value is "false".
If you do not want it to be a black box, just remove the attribute from the RTL.
Explicitly setting it to False or No in RTL is not supported.