Yup, that's just a marking dot on her knee. My Ai has them too. There should be a matching one inside the cup of the joint, and on the other parts along her leg and arm. As Linteia said, it's so you know the left part from the right part when the doll is unstrung. Trust me when I say that a left calf looks awfully similar to a right calf when the doll's in pieces.
I rarely unstring a doll to MSC and body blush, either, but if you do want to unstring, just take her head off by opening the head cap, and twisting the S-hook, (put the head where it won't get knocked over!) and then remove the S-hook entirely, easing the elastic back into the body until she goes slack....or yanno, just letting it snap back into the body like a rubber band. That works too... XD Then, pull the hoof/foot until you can get the S-hook out, on each side, and pull the elastic free. To remove the arms (which are strung separately), do the same thing you did with the feet.