Remove (bust out) the glass block, install permanent support underneath (plywood patch supported either by blocking between joists, or fastened directly to the underside of the slab with concrete screws), notch the edges 1.5" wide by 1.5" deep, pour a cement mix patch with standard aggregate, make sure you get out air bubbles as you go (no voids), finish as you please... you might look at a product line called Top-Cast for matching the aging on the existing, as far as that tan sanded concrete coating goes, maybe it's Rollerrock?... Hopefully you have a leftover can that the sellers left you.
You might should embed a tight grid wire mesh in there in the top 1.5 inch portion to help with any deflection.
If you do what others have commented, where you just fill the existing block with concrete, that will chip out. Concrete needs a minimum 1.5" thickness or it with break apart.
Also, it is essential to prevent any exposure to the elements with a cold joint. Water will penetrate and both leak into living space and freeze and break the concrete apart. If this is the case, you may want to pour the concrete with a removable 1/2" band between old and new to later fill with self-leveling waterproofing concrete sealant. You would use something like polystyrene (xps or eps) foam board that won't damage the concrete edges when you remove it.