Black specks in the hair does indicate that they likely have mites.
Please see a vet for diagnosis and treatment.
Please do not be tempted to use low dosed pet shop products or treat on spec as it can make it harder to deal with.
You will also need to throw their hay away, disinfect the cage and ideally cut their hair to remove as many as the eggs from the hair as you can
Please also see the guide below
1 Double-checking the gender upon arrival
- Sexing
- Pregnancies and surprise babies
2 When to quarantine new companions?
3 Vet visits
- Health check/registration upon arrival
- Reclaiming vet cost for newly bought ill guinea pigs
4 Common illnesses in new guinea pigs
- Ringworm
- Respiratory infection (URI)
5 List of guinea pig...