Hello, Welcome to HCM. I understand your problem. The pimple you have is a kind of big acne known a nodulocystic acne. Sometime they form when small pimples get infected by bacteria. After that, pus and blood may accumulate in that leading to increase in size. The purple black color you have observed inside the nodule is due to blood inside. I advise you should apply clearasil or any topical cream containing benzoyl peroxide on the acne prone areas of your face on daily bases. This will prevent development of black & white heads from which nodular acne can form. You should use anti-bacterial cream or gel containing clindamycine whenever you develop a nodular acne. You should wash your face with mild face wash 2-3 times a day. Refrain from applying hair oil and eating excessive oily foods. Take treatment of dandruff if you have. I hope this will be helpful. Take care.