What is the ruling on clothes that were washed in a public washing machine and a blood-like stain was found on them? Are they considered impure (najis)? And how can they be purified if they became impure? Is it sufficient to wash them again in the washing machine for purification, along with mentioning the views of the four schools of thought?

If you are unsure whether a spot is blood or not, the original ruling is that the garment is pure, and its impurity is not ruled based on doubt; for the original ruling regarding things is purity. If you are certain that it is impure blood, then wash only the garment that has the blood. The rest of the clothes are not ruled impure due to the small size of the spot and the fact that the washing machine water is not affected by it, unless you adopt the madhhab (definition) of the Hanbalis and Shafi'is, in which case all clothes become impure upon the meeting of a small amount of water with the impurity.