Most cases of keratoconus rarely lead to complete blindness.
However when you do have keratoconus it can degrade your vision quite a bit to where you can hardly see anymore which can be almost blindness completely.
If you get treatment soon enough for the keratoconus then you can usually restore your vision to normal or almost normal and lead a normal life.
But if you let the keratoconus progress and degrade your vision too much then it could lead to almost complete blindness.