I wouldn't worry about the dark spot on the shell; just might be her "signature"! If shell thickness is improving, that's great!! If they're still not thick enough, encourage more calcium intake by offering yogurt or cheese. Sometimes, they just don't much care for the oyster shell.
The easiest way to break a broody - put them in a dog crate by themselves for 3 full days. The bottom MUST be wire (what I did was took a plastic crate & cut the bottom out, and added hardware cloth), because that's what actually breaks them of it. THen, you have to either hang the crate up (some folks hang it right in the run), or put it up on blocks like cinder blocks. THe idea is that there is air flow underneath the crate. DO NOT give access to a nest box or anything, and no shavings or any type of bedding. Just make sure they have food/water. I use the small cups that are for parrots and hang them off the front of the cage. 3 full days, then put her back & she'll be done being broody!