My finishing pro suggested taping off the lighter area, use a Minwax WoodFinish stain to darken the lighter wood; while the stain is wet remove the masking tape and with a dry brush blend that straight line wet edge into the two sides. Then let the stain dry, perhaps 24 hours or what the instructions recommend, and evaluate. Better to have to apply the stain 2 or 3 times to sneak up on the desired result than try to hit the desired result in one step and end up too dark.
Minwax stain includes both dye and pigment with enough linseed oil to fix the dye and stain to the surface.
Again, experiment on scrap as much as needed.