If you want to make the line black at the time of plotting, add the argument 'k' to the plot command.
Example:
figure;
x = 0:0.1:10;
y = sin( x );
plot( x , y , 'k');
If you have a line that is blue and you want to make it black, you can use findobj to get the handle of the blue line and then set its color property value to black manually.
Example:
% plot something
figure;
x = 0:0.1:10;
y = sin( x );
plot( x , y , 'Blue' );
% the color values for blue and black
blue = [ 0 0 1 ];
black = [ 0 0 0 ];
% get the handle for the blue line
hline = findobj( gca , 'color' , blue );
% Set the line's color property value to black
set( hline(1) , 'color' , black )
