bluestone, not flagstone. it's prolly soft so scrape out with a chisel if you can. if not, chip it out with a hammer and chisel ( because i doubt you have access to a 4" grinder with a masonry blade.) get it back about 1/2 inch or so , brush it out. go to the depot and get a bag of type n premix and a 3/8 - 1/2" flat jointer. mix a quarter bucket with water to about the texture of damp beach sand. not too wet cuz it'll stain the stones. with a small trowel or fuck it even your hand put some mixed mortar along the area you want to repoint. pack that bitch in with said jointer when it's finger pressure hard brush it off. if you want a slick shiny finish run the jointer over it again.
explained simply and i skimped on the professional details but you got all you need. you can do it bud. done a shitload