Most places I’ve worked it’s fine to just comment. By approving or blocking, you signal to the team that you’re taking on the role of “the reviewer” for the code. Approving means that you have enough context to be confident in the change and stake your reputation on it. Blocking means you commit to doing subsequent rounds of review and eventually approving when your concerns are addressed (or you’re asserting a firm preference that this change should not happen).

Maybe you’re not the best person to approve the code, or you just don’t have time to commit to the reviewer role at this second, but you’ve still spotted something that you think will help the person writing the code; so, leave a comment.