Thank you. I have changed the default object preferences to black but the skecth lines still are shown in blue. Maybe I am doing something wrong, I will keep trying.
If you did change the object creation preference this will only effect new sketches any that already exists. If you change that then you new sketches will have black lines moving forward.
There are several settings that affect the color of the curves in a sketch.
By default a sketch uses system defined colors. The colors are different when the sketch is active or inactive. These color are defined per part and can be changed in preference sketch.
When you want to use your own colors then you can change the sketch to allow display of object colors. You can find the option to turn this on in settings. MB3 (right mouse button) on a sketch in part navigator and then choose settings. Now all curves are displayed in the color you created them.
This brings us to the color of the curve. You can change the color of the curve using MB3 and then 'edit display' on a selected curve. By default all colors are blue, this is the color that is defined in menu/preferences/object. The preference is for new objects only.
On top of all this in drafting the default display is monochrome. As soon as you make the sketch inactive (press finish), the sketch will be black. You can toggle monochrome display on and off using preferences/monochrome display.
Now there should be no more secrets on sketch curve color.