Hello,
It's not clear to me what you mean by blue black on your thighs or knees. Are these skin changes or bruises? My best guess is that you are noting some darkening of the skin related to relative increases in estrogen and progesterone at that time of your cylcle. We sometimes see a similar phenomenon in pregnancy. If they don't bother you than there is nothing to do. If you want more information I suggest you see your ob/gyn when they occur.
Best of luck,
Dr. Downing