Welcome to the forums, John!
It looks from the photo, that the sod adjacent to the bluestone is already lower? If the bluestone is sloped toward the grass, and the grass also continues the slope away from the bluestone as you claim, I don't know why there would be water pooling at all.
Did the sod near the bluestone sink to create a slope back
toward the bluestone, thus catching the water?
The general grade needed for hard surface water runoff is less than what is needed for any soft surface (i.e., a lawn in this case). This is because it is impossible to grade any earth surface at such a shallow grade, so that after settling, every square foot drains as desired. I'm sorry you read somewhere that an equal minimum grade for hard and soft surface water runoff is acceptable, but I would consider throwing out that book. Landscapers will put in a sharper grade, at least for a short distance, when they know excess water will exit to that area. Ummm..... doesn't that make sense?
Sorry to be so harsh. I know this isn't a namby-pamby answer, and I'm just not good at beating around the bush. Unless I am not understanding the problem correctly (which could be very true), it seems that it is the design or the installation of the sod (or both) that is at fault. If you can fix that, there will be no need for a French drain. Natural drainage is always better, longer lasting, more environmentally friendly and easier to change in the future, than any French drain.
