I have found polyester resin to the be ideal for this kind of repair.

It is very strong in it's cured state, it seeps into fibers and produces a strong bond (it's intended use is laminating wood and resining fiberglass) and it is slightly flexible, so it will move with wood as it bends and expands with humidity.

I prefer jet black for most wood repair work. I have given-up trying to match wood colors when disguising a crack.

Specialized pigments are available for resin, though I have found almost any black coloring works. Here I am using black airbrush paint. A little bit of it goes a long way.

Tape-up the sides and bottom of the piece to keep the resin inside. Cracks run deep, so it is sometimes wise to tape the entire piece inside a plastic bag.

Do the repair work before making any cuts or planing on the wood and don't worry about the resin puddling on the outside.

Pour it in the cracks until it overflows.

Just to keep everything within the confines of the wood, I taped a bag around it.

You got resin everywhere. It's a good thing we used the plastic bag.

Polyester resin can be worked with most woodworking tools.

Be careful if you choose to go at it with a chisel, the excess resin will throw-off glass-like shards when impacted.

A bandsaw will cut easily and safely, then the resin can be planed, routed, or sanded. This is a structural repair, so the block is now fit for any purpose as a solid piece.

To simpler woodworking, Walter Smith