There are black solid things inside little holes around my penis which is visible if the penis is erect. what are these?

Blackheads-comedos: Hightly likely, what you described represent the presence of blackheads-comedos resulting from dried-up secretion plugging up sebaceous glands, like those on many people's face, esp. the forehead or back, which are harmless, become more visible at thinning-out penile skin from erection, & need no treatment if mentally & physically acceptable to you. If not, they can be popped out manually or tool.