For Online Computer Support, Ask a Computer Technician
Hello and welcome to JustAnswer, I look forward to assisting you today. Feel free to disregard the Premium Service Offer. That is an automated popup initiated by the web site. Would it be possible take a picture of the monitor and send it to me?
I would be glad to help. It sounds like you may have a dead pixel. Is the black mark very tiny like a spec of sand? I've attached an example image of a dead pixel: