The Black Heart event needs a few tweaks before the update goes live.
1) The Black Heart researcher should NOT spawn within the Black Heart Territory's maximum size. The researcher will be killed before you even get a chance to speak to them about the heart's weakness.
2) Place a check on whether there are already zombie buff curveballs active. My black heart event started right after a sentinel mutation took effect, so black heart zombies were sieging my base nonstop because my base happened to be within the black heart territory. Stealth wasn't an option, and the black heart researcher was killed almost immediately after she rendered. They could see us at all times and I couldn't move vehicles in front of the entrances because of the black heart's effect on vehicles, so I basically had to run away from base to get my other survivors out of render distance before they all died because survivor AI can barely defend itself as it is.
3) At Lethal difficulty, the Black Heart's territory expands at such a rate that you might as well have it spawn at maximum territory size right away because on my playthrough, it literally took up 1/3 of the map in 45 seconds. Why bother have it even expand if it takes less than a minute to reach that size? At least give players a chance at launching a strike before they have to trudge 500 meters on foot just to get to the thing.
4) The reward for killing the black heart is really lame. 2 rucksacks is not enough of a reward, you should at least be guaranteed a rare weapon or high-end facility mod in the loot.
In all honesty, the Black Heart feels more like what should happen automatically when there's only 1 plague heart left on the map, it has much more 'final boss' energy about it. After struggling through it (It took 12 meds in plague cures and 2 heavy weapons to take down) killing the other plague hearts afterwards felt incredibly simple and underwhelming.