Black streaks in phlegm. Should i worry?

Hello doctoe. I was covid postive in may and recovered with at home with mild lung pneumonia. I was also on blood thinners and stopped post the D-Dimer came out normal. Post covid I had no cough or cold however had phelgm coming and continue to spot black streaks in phelgm. This is not always and mostly just once in the morning. I had consulted a pulmonologist and was asdvised to take cbc and chest xray. He asked me to take CT only if i have cough which is not the case. Everything is normal in the xray and blood reports. However I can continue to see the black streaks in the phlegm. Is it a common post covid reaction? When should I worry? Are there any further diagnosis required. Pic added for attention. Please suggest. Thank you.