When i blink looking at white wall i see black veins in my vision. also when i wake up in the morning for few seconds when i blink it looks more prominent. what is it? i'm 21 years old. is it normal?

What you're describing is almost certainly a very common (and normal) experience of being able to see your retinal arteries and veins. The retina (analogous to the film in a camera) has arteries and veins coursing through it. Under most circumstances, the brain suppresses those images, but some people in some situations see them and it's just fine!