How should I refer to myself (single author) when addressing reviewers comments?
It's up to you and it doesn't matter much. Even if the process is double blind, it does no harm for the reviewers to know there is a single author or, conversely, to think there are multiple authors if you use "we". However if you don't consider yourself a "we", use the singular I. Exclusion of "I" in academic writing is absurd in all circumstances and while this isn't a place for that particular trench war, the comments letter is directly from you to them and "I" is appropriate in such cases.