Hi.. Your doctor who has examined you may be right.. It may not be an STD infection.. Masturbation can cause dark blister kind of lesions to develop over the glans of the penis... If you are still not convinced the only way of getting over your fear is by checking the tissue under the microscope for presence of infection.. All the best...
Thanks for the reply. However this darkness has been there for quiet a long time (more than 2 or 3 yrs) and now it looks like it is kind of differing in the tissue surface.