I was looking for a way to potentially counter a medusa's petrifying gaze when something hit me: Why not cast the 5th level spell Seeming on her face, giving her a blindfold or something so I can no longer see her eyes? This idea opened up a bit of a can of worms, however.
Looking at the description of Seeming, it does not have the familiar stipulations of most illusions, like Major Image or Minor Illusion:
…If a creature discerns the illusion for what it is, the creature can see through the image, and its other sensory qualities become faint to the creature.
The changes wrought by this spell fail to hold up to physical inspection. For example, if you use this spell to add a hat to a creature's outfit, objects pass through the hat, and anyone who touches it would feel nothing or would feel the creature's head and hair. If you use this spell to appear thinner than you are, the hand of someone who reaches out to touch you would bump into you while it was seemingly still in midair.
A creature can use its action to inspect a target and make an Intelligence (Investigation) check against your spell save DC. If it succeeds, it becomes aware that the target is disguised.
Nowhere does it say that the illusions become "faint" for the viewer or that they can see through them – only that they are aware that the target is disguised. Furthermore, Seeming only fails to hold up to "physical inspection", which the description elaborates as touching and feeling. This leads to a few questions:
What does "becoming aware that the target is disguised" entail? I assume the language for Seeming is different than that of most illusion spells for a specific reason and thus, these illusions do not become necessarily faint, RAW.
If I cast Seeming to put a blindfold on the Medusa's eyes and she fails the Charisma Save, am I immune to the Petrifying Gaze effect? Are others immune to the gaze effect if they choose not to make the Intelligence check?
Is the Medusa able to see through the blindfold? What about after succeeding the Intelligence Check to "become aware of disguises"?
If #3 is a no, this becomes a very powerful spell. Par Seeming's description:
This spell allows you to change the appearance of any number of creatures that you can see within range.
Does the poor wording of this spell then become Mass Blind on a Charisma Save?
Obscuring, yes. Blinding, unsupported.
By RAW, the obscuring a Medusa's Eyes Functions as Stated.
A strict interpretation of "aware" in the event of someone passing the intelligence check when viewing the illusion supports the use of seeming to block the view of the medusa's eyes. The perceiver is aware that there is an illusion, but that no text in the description indicates effect beyond the awareness. So the illusion obscuring the medusa's eyes is still there, but the someone knows it's an illusion.
Blindness Unsupported by Text.
The spell does not state one way or another that it obscures the senses of the recipient of the disguise. Inferring that it can be used as an actual blindfold is not directly supported by the text.
Spells Do What They Say They Do
The spell description does not state it has the capacity to incur the blinded condition on recipients.
Blindness by Illusion is Paradoxical
As pointed out by guildsbounty, if illusions could blind the recipient, it would make many uses of them blinding.