There are a couple of ways to do this. First, in the Inspector with nothing selected look at the top title bar above the icons and notice it says "Inspector: Default Settings". Click on the "Pen Properties" icon and set the color to black. Black is now the default. A second way is to right click on a line and in the popup menu set the color to black, this only sets the color of the selected line to black so right click again and this time click on "Set Default" in the popup menu. This set default will work with any setting that has a default.