createjs.Shape()
with graphics setStrokeStyle(1)
draws a 2px line instead of 1px. If you screenshot the JSfiddle output below, you will seee that the line is 2px tall.
How do I draw a 1px line?
The line that you draw in going in between two lines of pixels, so it's coloring both sides of it in grey instead of black. change your code to
line.graphics.moveTo(20,74.5).setStrokeStyle(2).beginStroke("#000").lineTo(280,74.5);
in order to draw over only one line of pixels.