In this poem, there are two words (one each in the stanzas about the mole and the owl) that reinforce to us the idea that these animals are blind (at least to our way of thinking).
In the first stanza, we are told that the mole "groped for worms." The word "groped" reinforces the idea of blindness. It does this because to grope is to feel around for something that you can't see -- you grope for your glasses in the dark, for example. Since the mole is groping, it sounds like it's blind.
In the stanza about the owl, we are told that the owl "blunders." To blunder is to stumble around in a confused way. If you were blind, you would blunder around because you would be unaware of what was around you.
These two words help to reinforce to us the idea that these animals are blind.