Mistake #1 - Wearing the wrong foundation. Either the wrong color or the wrong formula. Foundation is only meant to even out your skin tone. Use as light a weight foundation as can do the job and make sure it perfectly matches your skin on your neck.
This totally depends on what kind of eyeliner you are wearing and where you are placing it. If you do a soft and smoky black line in the right place for your eye shape, then yes, you can wear a black liner.