The following factors could cause dark cuticles:
• Dehydration
• Vitamin B deficiency
• Environmental factors such as excessive cold or sun exposure
• Excessive washing of hands with soap/detergents
• Unhygienic habits like biting nails and sucking thumb
• Some medical problems, such as fungal infection, heart illness or kidney issues, skin cancer, and so on, can cause the skin around the nails to darken. In such circumstances, seek medical advice to address the underlying issue.