If your skin gets too much sun exposure then yes the over exposure to the sun when tanning in the sun can eventually lead to permanent darker skin.
So if you tan a lot in the sun you could have permanently darker skin as a result of over tanning in the sun but it won't happen right away.
It can take years for the sun to permanently darken your skin from tanning but it can eventually happen.
However if you're experiencing permanent darkness of the skin from suntanning then it's likely a sign that you're getting too much exposure to the sun and are causing bad skin damage so it's best to avoid tanning too often in the sun.
You could actually get skin cancer as a result of over tanning in the sun so be careful.