Line 63 says that the fire of hell has no light, but "darkness visible." this phrase seems to contradict itself. what do you think milton means?

Milton is using the nature of darkness to indicate evil, the opposite of God (or "light").

For this reason the fire of Hell has no light. (God and heaven, however, are always described in terms of light.)

As a result, Milton's vision of hell is so dark that it is "darkness visible" -- meaning it is visible evil. In forsaking God, Satan and his companions have forsaken all light as well.