Depends on which dolphins you're talking about. Most dolphin species are not blind. But there are river dolphins who live in very murky waters, where sight isn't particularly useful.
And of course there might be individuals that have become blind through injury or disease.
No Dolphins are not blind. However they use sonar to judge what and where objects and other animals are from far away. They also use this same sonar to communicate with other dolphins.
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Yes, they use their white bellies to blend in with the sun light and their dark backs to blend in with dark depths.