Do you have scraps to try stuff out on? That tile, as I recall, is somewhat porous.
Try the clear tile sealer first. It will darken up the tiles.
You may want to also try some home brew things to darken it up. This is presumable a nook where there were shelves so traffic wnt be a concern.
First thing that comes to mind is Boiled linseed oil. It is cheap. Its used in woodworking. No idea if it would be compatible with another tile sealer, just figure it wouldn't be. It will darken things a little more than a clear sealer will. (Again, assuming the tile is porous).
You could also try some regular ole wood stains, coffee, tea or anything else to give you the color.
I suspect that a regular tile sealer will get you pretty close. Try that first.