The following article contains spoilers from Nightwing #96, on sale now.
Nightwing's war against Blockbuster took a dark turn when his secret identity was exposed to him. However, Blockbuster was killed shortly after by Heartless at the end of Nightwing #96 (by Tom Taylor, Bruno Redondo, Caio Filipe, Adriano Lucas, and Wes Abbott). While this seems to put the matter to rest, there is still a possibility that Nightwing's secret is out there somewhere, a ticking time bomb that could go off at the worst possible moment.
Blockbuster is dead, no one is denying that. However, while his empire may have been crumbling around him, he still had a network at his disposal. In the time between him fleeing and Heartless killing him, there is a possibility that he told someone about Nightwing's true identity. This isn't even factoring in the DC Universe's habit of resurrecting villains. Blockbuster may very well come back to life at some point with his enemy's greatest secret.
By the end of the story, things were definitely not going Blockbuster's way. Oracle's revelation that he had been running one of the city's worst private prisons, profiting off the misery of the criminals there, effectively turned his entire empire against him. The thing is, this was only a local response. There is no way to tell how quickly or slowly this news spread.
It means that Blockbuster still had some people who were loyal to him before he was killed. They likely wouldn't be loyal for long, something that he knew, but a desperate man will turn to even the smallest sliver of hope when cornered. He knew there was no way he was getting out of this without help, so he would likely call upon the vast network that was still somewhat at his disposal, possibly leveraging Nightwing's identity in exchange for help, or perhaps even telling them outright as a means of petty revenge.
Unfortunately for him, Heartless' arrival stopped any chance he had of seeing his work through to the end. This creates another possibility -- Heartless was tracking Blockbuster through the streets. If Blockbuster told anyone the truth about Nightwing, then Heartless could have overheard it. It's already been shown that Heartless stalks Dick Grayson, but knowing that he and Nightwing are one could change the landscape of their battle entirely.
He's been a background player for a while now, but with Blockbuster gone, he seems intent on claiming the city for himself. That of course will put him on a collision course with Nightwing, and knowing his secret identity would be invaluable to destroying him. Then again, there is always the chance that Blockbuster will be resurrected. Villains have an unfortunate habit of coming back to life, if he comes back, he might be able to end Nightwing once and for all.