There are two main reasons that your hair may look darker if you use shaving as your main technique of hair removal.
The first reason is the reflection of the light. The hair naturally has a tapered end, and there is no cross cut at the top, so it looks very smooth at the tip; however, if you cut your hair with a razor, you are making a crossed edge on the tip of the hair that exposes more pigments and makes the hair look darker.
The second reason is repeated trauma to the skin. Dull razors scratch the skin, and the natural reaction of the skin to the trauma is to make more pigmentation to protect the skin against the hazardous effects of UV and sunlight. In medical terms, this extra pigmentation is called “post-traumatic hyperpigmentation.” In this case, the hair is not darker, but what is really darker is the skin that makes the hair look darker than normal.
Shaving by itself doesn’t do anything to the pigmentation and thickness of the hair, nor does it change the type of hairs from vellus to the terminal.