Have you done the deed yet?
If not, show both ends.
I've only sawn a few small logs on my vertical bandsaws, but have resawn acres of lumber.
Once you figure out the orientation of the first cut, run the log over your jointer to make a flat on the "bottom" at 90 degrees.
Even if you have to take 1/2 or 3/4" depth in a few passes,, it is not really removing anything useful, and it will stabilize the log immensely for the actual cut.
I would be very tempted to put a flat on it, or to screw a flat board to the "bottom" as the sliding surface, and saw it free hand to a chalk line. for the first face. Reason being 1.)what muskyed said about all bandsaw blades lead. Freehand, you will just follow the line. If the log is constrained on a guided carriage and the band leads, it will wander and under cut, or cut a bow, or all three until it runs out or stalls. Be sure there is enough flat on it that the log can't roll, though. 2.) It's not a big factor for air dried wood, but in many cases as the cut progresses, lumber often starts to spring from release of stress. Freehand, you can sense this more quickly, and adapt. In some cases stop the saw, wedge the wood, and back out to try from the other end.
OTOH, if you do choose to screw a board to the bottom & use that as both a flat and a guide, you are probably familiar with how most people find the lead in a new band. Use a flat board with parallel edges; and with a marking gage, draw a straight line parallel to one edge. Then saw it very steadily freehand, stop in the cut, and clamp the board. The parallel edge indicates the alignment to the table, to place the fence. Draw a line on the table, using the edge of the un-moved board as the reference. I then compare that to either a table edge, or one of the T-slots. E.g., is the front of the line 1/4" closer to the T slot at infeed edge compared to the outfeed edge? I use that relationship to set the fence at any other space from the blade. Set the fence, then saw some boards relatively fast, and see if the band tries to climb outward and take an ever bigger bite, or if it leads toward the fence. Make very slight adjustments until the band tracks right down a parallel line. with a board against the fence. Then start sawing the "good" wood.
Don't cut too close while slabbing. wood moves, sometimes the saw band does too. Trying to save an extra 1/8" of cut might lead to a board that is undersize after you flatten/straighten it. If you have a log to practice on, this can all be dialed in on the saw and process. But if it is one shot, you have to be a little conservative.
I'm assuming a saw with a big enough table, and adequate support. If you are sawing logs you probably have a 30" or 36" saw. Another assumption is that the guides and wheels are set up correctly so there is not induced twist or bow in the band. For instance a band that is riding the back guide heavily before it even begins to cut, is bowed to start, and then will wander as the cut progresses. A band that is constrained to deflect by a mis-adjusted side plate of the guides will try to make an S curve, and wander in the cut vertically. A band should ideally be a couple thousandths from each side plate and from the back guide when running without load. Not rubbing anywhere except sometimes the weld area, when turned through by hand. Barely kissing any of them as it idles under power. Meaning the wheels have to be round, too. A throbbing band from slightly out of round tires does no good for cut quality. etc.
My memory is that blackwood can be fairly hard on a sawblade. Start with a sharp one. My buddy who resaws exotics (rosewoods) more often than me uses carbide tipped bands. In that species, I've only resawed short 8/4 x 6" - 7"wide boards, a few at a time.
Good luck!
smt
