Dark sand like substance in laundry tub I think it is from watewr softener

Is it redish. Look very very closely, is it like really, really small balls, really smooth and slippery? If so, it is media from the softener. You would need to have the softerner seviced or replaced.

If your on well water? You maybe getting sand at low water, if not well water then it mite be rust from pipes if new pipes ?or how old is washer tub it could be a seal.I am not sure with out seeing sand.

Do you have a hard water faucet before the softener you can check? Black sand in the water generally comes from your well. Usually a good working softener will stop most of this in the mineral tank. If you are seeing this only in the wash tank, it is likely happening only during softener regeneration. Remember, the average softener regenerates with only [you guessed it] hard, unsoftened water.

You can install a sediment filter before everything but it will likely fill up pretty quickly. But, on the plus side, it will keep your brine suction screen from getting plugged with sand. If your screen is easily accessible to clean out, then just let it be. But if it is a royal pain [having to remove the timer, etc constantly] then I would consider the sediment filter.

T'care,
Mike