Are baby platypuses born blind

No. Platypuses are not blind. In fact, they have acute vision. However, they do close their eyes when searching underwater for food.

They use a form of electric sensing, called "electroreception", whereby they detect changes in the electric field generated by their bill. The presence of something within that field would cause fluctuations in it, and so the platypus can 'see' the prey by electricity. They close their eyes so as to be able to use the electric sense more efficiently, in the same way that closing our eyes often helps to hear better - it removes confusion.

The position of a platypus's eyes enable it to scan the banks of rivers and creeks to watch for potential predators. Because platypuses' eyes contain cone receptors, it is believed they can also see colour.