A third level High Elf Pact of the Chain Warlock with an Archfey patron casts the find familiar spell and then chooses a sprite form for the spirit to take.
The Warlock commands the familiar to enter a dark cave, and can see with darkvision through the familiar's eyes. The sprite does not have darkvision and, therefore, either must have a light source or be guided by the warlock's commands (such as "go right 10'"). The sprite is blinded by the dark.
This was the implementation of the rules by the DM in the game situation which I agree with. After checking the Player's Handbook, I can't see any argument against this ruling. I want to keep the sprite familiar, and I rarely use the familiar in game, I keep it for specific situations, one of which could be the "dark cave" situation.
Is there a way for the sprite to gain the senses of the Warlock such as darkvision?
My question is about the possibility of a Plan B to enable a "ninja sprite" in this situation, either now or in the future.
Include abilities, magic items, feats and spells which can be accessed at higher levels (including multi-classing) that might help, and also include tactics for different ways to use the familiar in this situation. I would prefer to exclude the option for the familiar to take another form such as pseudodragon to avoid the "choose a different familiar" option for role-play reasons.
The DM already ruled on the question posed here, so I would need some more evidence before going to "ask the DM" again about the ruling. The best answer could be: "No, there is no Plan B, your sprite will always enter a dark cave blinded, go with it and enjoy the challenge!".