Don't look for a word, look for an agreement with your DM
I am looking for a way to get someone to temporarily blind themselves. Yet, for the life of me I cannot find a solitary verb that means to close your eyes. Funny how bad the English language is
But Command says (emphasis mine):
You speak a one-word command to a creature you can see within range...You might issue a command other than one described here. If you do so, the GM determines how the target behaves.
You may be under the impression that if only you can find the 'perfect word', you can force the DM to impose the blinded condition to a foe on a failed save. That's not how the spell works. There are no 'hacks' to "Rule 3: The DM Narrates the Results".
Consider the "typical commands and their effects" in the Command spell - Approach, Drop, Flee, Grovel, Halt. Note that none of them have to be ruled the way they are by the nature of the word itself; rather, the game designers decided on an outcome that was appropriate for the power level of the spell and then used them as model examples for DM's and players. Suppose one of these specific words had not been included in the spell description, and a player was presenting it as a novel word "other than the one described", but the DM considered the suggested effect too powerful:
Player: "I'd like to use Approach to have the target move toward me by the shortest and most direct route, and end its turn if it moves within 5 feet of me."
DM: "Command allows a single word only - what you are describing is "Approach (me)". How does the target know who or what to approach? And why do they end their turn? They have only used movement and have their other actions."
Player: "I'd like to use Drop to have the target drop whatever it is holding and then end its turn."
DM: "Command allows a single word only - what you are describing is "Drop (things held)". How does the target know not to "Drop (self)" and fall prone with their items still in hand? And why do they end their turn? Dropping things is a free action - what you are describing is more like 'Drop and Halt'."
Player: "I'd like to use Flee to make the target spend its turn moving away from me by the fastest available means."
DM: "Command allows a single word only - what you are describing is "Flee (from me)". How does the target know not to Flee from one of your teammates, or someone in their own party, or their home?"
My point is that none of these words inherently force the condition on the target by the nature of the word itself. Instead, they were considered appropriate effects and then the word was chosen as representative.
As a further consideration of how your well-intentioned search for the 'perfect word' is actually misguided, consider that you admit that finding such a word is difficult due to the deficiencies of English. But (unless you are in a very specific campaign), the word actually used by your caster is not in English - it might be in Common, or Gnome, or Draconic. Under Rule 1 ('the DM describes the environment'), it is the role of the DM to decide the vocabulary of these imagined languages within their campaign world. So, even if you could find a 'single word' in English to use as a Command for someone to close their eyes (and I would suggest squint or blear), the DM is well within their purview to say that such a concept does not exist as one word within the language the caster is actually using in the game.
So, rather than looking for a perfect word, first go to your DM and ask whether it would be acceptable to have a Command result in the Blinded condition. As you note yourself in comments, Blinded is strictly superior to Prone for your use, so your GM might consider that too powerful an effect.
If your DM does approve the kind of effect or condition you are looking for, choose any word that evokes that feeling, but realize that the word itself is just for descriptive flavor, not inherent power. The DM might even say, "Okay, your command is 'Blind yourself' but don't worry that it is two words in English; it is only one word in Common!"