Hello,
First of all no need to worry if you don't have any other symptom of covid19. Now coming to your problem, blister on toe is usually seen due to constant friction on one area due to hard or new footwear. This blister usually lasts upto 2 weeks and ruptures on its own releasing the clear fluid.The picture you have sent shows it has healed almost completely so you are experiencing itching on that area and it has darkened (post inflammatory pigmentation).
Next Steps
At this stage you just need to apply moisturiser for feet ( Xerina cream) twice daily after cleaning with warm water .