I was rereading the manga the other day and I saw that Mamoru, after Sailor Moon was able to undo his brainwashing by Beryl, was blind when he woke up. That gave me a "huh?" moment. We never see his eyes get injured, so I have two theories: 1) Queen Metalia is corrupting the Earth, and this acts like a poison of sorts, robbing Mamoru, who is deeply connected to the planet, of his sight. Theory 2) is that it's specifically Metalia's darkness, as it engulfs the Earth, also clouds Mamoru's vision because of his ties to the planet. However, for this last one, one could argue that darkness befell the planet practically during every final battle Sailor Moon has at the end of an arc, but this is the only time Mamoru looses his vision.

Honestly, I feel like both theories are a little weak, though the second one makes more allegorical sense. :\

You pretty much hit the nail on the head, though I think part of that was supposed to be because Sailor Moon was supposed to end early, but the popularity and all that pushed Naoko to continue the series.

It also could have been because he was that badly possessed by beryl, though it makes little sense considering how many times he was brainwashed later.

I always thought it was closer to Theory 2. In that whole part of the Moon vs Metalia battle, theres a lot of emphasis on learning more of Mamoru's connection with the Earth. Right after Moon defeats Metallia, Mamoru is able to sense where the rest of the Senshi are just by touching the Earth. So i think this theory that due to his closeness with the Earth and Metallia almost nearly engulfing the Earth in her darkness works to explain his blindness.

Small Lady wrote:

I always thought it was closer to Theory 2. In that whole part of the Moon vs Metalia battle, theres a lot of emphasis on learning more of Mamoru's connection with the Earth. Right after Moon defeats Metallia, Mamoru is able to sense where the rest of the Senshi are just by touching the Earth. So i think this theory that due to his closeness with the Earth and Metallia almost nearly engulfing the Earth in her darkness works to explain his blindness.

I also favor theory 2, but it seems with each arc, similar circumstances occur, yet this is the only time we see Mamoru loose his sight. Maybe as he gets stronger with each arc the dark energy of which ever enemy they're fighting affects him less and less?

Maybe so, Black Moon, it was in the future and he was brainwashed when it was really bad... Infinity, well, i dunno how to explain how he wasn't affected there other than Naoko just forgetting that? And then we have the sickness in Dream which affects him a lot due to its affect on Helio, Elysion and the Earth.

Stars he just gets straight up killed before even getting the chance to be sick or injured from Earths pain. xD

Number 2 definitely makes more sense.  Remember in SuperS(Dream Arc) Helios did explain that what happens to the earth would happen to Mamoru, and in the manga, during the Dark Kingdom arc the earth was being corrupted and so it would affect Mamoru.