Entropion is a disease of the eyelid. In itself, it does not cause blindness. The inward rolling lid can over time cause damage to the cornea (surface of the eye). This damage to the cornea is the primary reason it can be painful.
Certainly blindness could be a result of chronic damage and scarring to the cornea.
If there is untreated entropion, even in a blind eye this could be a source of discomfort. The most practical solution in this situation might be enucleation (removal of the eye), in that this would prevent any long term discomfort from the entropion and eliminate the need for eye drops or future surgeries. Eyelid surgery to correct the entropion may also be possible although in a completely non-visual eye I would question this approach.
Sometimes entropion may occur due to a secondary condition, such as a congenital problem, as it sounds in this case. For example, a dog born with microphthalmia (one or both eyes smaller than normal) might have some degree of secondary entropion. In this case, the blindness would less likely be due to the entropion, and more likely due to the microphthalmia. In these types of cases, again if there is complete blindness along with discomfort or corneal damage from the entropion, then removal of the eye is likely the most practical solution.
I would encourage you to talk to your vet to better understand the reason for the blindness, as well as the reason for the entropion (they may be different reasons). If you do decide to adopt, you could also get an opinion from a veterinary ophthalmologist to see whether they recommend any treatment.
