Are you trying to grow it indoors? That would account for the etiolation and yellowing. Too little light. You might also be over watering. The black spots look fungal and would be encouraged by poor growing conditions.

Yes, if it's indoors or not makes a difference. Most of my mints visibly start to turn as the weather turns- but mine are outside. If yours is inside looking like that, you want to improve your lighting and air flow, might need to feed it.

Watering might be a thing depending on how much water you are giving. Though they like moist soil, sometimes twice a week watering can give you soil that is too wet. Too much wet can lead to root rots and fungal issues on the leaves.