Is this onance or alliteration?

The phrase exemplifies onance because here, the "ear" sound is being repeated at least three times. The difference between alliteration and onance is just that onance has to do with vowel sounds and alliteration consonant sounds. So an example of alliteration would be "seared, soared, sung"-- notice that the vowel sounds are changing but the "s" sound is constant whereas in your example, "bleared" breaks the trend, but the "ear" sound is constant so it has to be onance.