Ebony timber mate would work in with the dark grain plans as a filler however Basswood has very little open grain for TM to penetrate and assist with grain popping. Black fountain pen ink may be the best bet but be prepared to buy quality stuff and wipe as much off immediately after application, and then next day wipe the remainder off with a slightly damp rag. You may need to do this a couple of times to make any impact and then lightly sand to knock down the furry timber fibres and expose some of the blonde timber underneath in preparation for the emerald green stain.

BIG TIP: Basswood staining = no higher than 180 grit sanding and even that can sometimes be too smooth as some of the top or bottom slab sides can be very shiny straight out of the box and once this happens it tends to repel any staining attempts or make things look like there are glue spots where none existed.