I don't know Tasmanian blackwood at all I'm afraid so can't be specific about that, but all wood has the potential to crack in this way. This doesn't mean the drying was carried out poorly, although an excessive tendency towards this would generally indicate this sometimes it's just inherent to the wood (sometimes just the individual board).
It's not what you asked about but as you included details of your sanding I wanted to comment on a few aspects.
sanded the body edge with 80 grit at the advice of a cabinetmaker
80 grit is not a finishing grit, it should never be used unless you're dimensioning or shaping wood. I presume what you were doing would be classed as a finishing operation and unless something unplanned for has occurred you should never need to resort to 80 when finishing.
100 is about the coarsest you would normally need to use and frequently 120 is the starting grit of choice, coarse enough to do what's needed not too slowly while not leaving excessively obvious scratches.
Obviously the quality of the routed surface is a factor here and routing operations, if carried out properly, should leave a surface good enough that only minimal sanding is required. If not, look at your router speed and feed rate, as well as the sharpness and design of the bit.
You can nearly strive to have the routed surface be finish-ready. A quality router bit that's very sharp is an important starting point for this but doing the lightest of skimming passes as a last step should always improve results. One way of doing this is by running tape around the template, or on the router's fence, when doing the major shaping and then remove it for one final skimming pass (taking off wood equal to the thickness of the tape used).
with the view to move up in grit to 320
Consider sanding to a lower grit on the face grain (220/240) and a higher grit on the end grain (400 or even 600), to help even up the colour once finish is applied. Sand the end grain first, then do the face grain as a final pass sanding by hand using a block.