It depends on how you set up your dimension lines
How is your settings now?
Can you post a sample drawing and pdf to identify the issue?
If you change the pdf settings to be in black and white then all lines should print black. If you need some color then you can seleect all of the dimensions and then change the color with the Line Color tool. Or to permanently make your dimensions darker you can change the color in System Options > Colors. I am assuming you are referring to driven dimensions that show up gray by default, so the color for those dimensions is listed under "Dimensions, Non Imported (Driven)".