Brief Answer:
Due to improper oxygenation and circulation

Detailed Answer:
Hello,
Thank you for your query.
I'm Dr. Shuba Hariprasad and will be answering your query today.

Purple or bluish fingers (cyanosis) is due to lack of oxygenation.
This could be due to a heart disease, lung disease or circulatory disorder. Even an airway obstruction can result in these symptoms as does muscle weaknesses due a stroke

*did he have any history of heart or lung disease?
*did he have any chest discomfort or breathing difficulty?
*was he a diabetic?
*does he smoke or consume alcohol?
*any other medical issues or medications?

I recommend :
-a physician, intensivist and cardiologist consult.
-an echocardiogram and electrocardiogram.
-immediate restoration of oxygenation. He may need ventilators support and intensive unit care.
-arterial blood gas analysis.
-chest x-ray
-blood tests (complete blood count, blood glucose levels, kidney and liver function tests)
-urine analysis

Based on the clinical findings, test results a treatment plan will be initiated.

Hope I've answered your queries. Please let me know if there is anything else I can help you with.

Wish you both good health.

Regards,
Dr. Shuba Hariprasad

---