It states that the Mariner blessed it unaware but I'm not sure about that.
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The Mariner sees the water snakes as "slimy things," ugly; this is also the way he sees himself after killing the albatross. The fact that he can now see their beauty is a blessing to the snakes...... it is also a blessing to the Mariner because the guilt falls away (the albatross).
Rime of the Ancient Mariner