How to darken flagstone edge on fireplace hearth
You darken flagstone with sealer/color-additive.   Or you can try heat.  I have darkened flagstone outside using a blowtorch, it works.   I am not suggesting that you do this as the results will vary and the sealer may melt weird or possibly start a "small fire".   The method does work as I have used it for outdoor grills and fire pits but saying that I would not have practiced my first time inside.   

There is no quick fix for this.   You have to get sealer off first no matter what.   Even with past experience no way I am torching something with sealant on it.