It is generally not recommended to use black fiberglass tissues in a dishwasher. Fiberglass is a type of plastic that can be damaged by the high heat and water pressure in a dishwasher. Additionally, the black color may be due to dye, which could potentially leach out and stain other dishes. To maintain the integrity of your fiberglass items and the cleanliness of your dishwasher, it's best to wash them by hand using warm, soapy water.