Changing the identifier/marker type a Legend

I am trying to adjust the identifier/marker type in a legend. I will illustrate my issue through an example: Consider the following figure:

if true
   A = [-1 0;1 0;0 -1;0 1];
   b = [ 1; 1; 1; 1]      ;
   plotregion(-A,-b,[],[],[1 1 1],0);
   legend('box')
end

You will see that the identifier in the legend for the plot is a black box. Is there way to change to a black line as in the following example?

if true
  x = [1,2,3,4,5];
  y = x ;
  plot(x,y,'k')
  legend('A line')
end

I checked all the properties of the legend command, and could not find a lead.

Thanks,

[Jan]

plotregion does not belong to Matlab's toolbox function. Maybe you mean: https://www.mathworks.com/matlabcentral/fileexchange/9261-plot-2d-3d-region. Please do not let the readers guess such details.