A spin-off could potentially be a bad idea for the show, depending on what it focuses on. If fans of “The Blacklist” feel like the new show already covers the kinds of topics they were looking for in the original, they might eventually abandon it.
It also depends on what kinds of characters will be developed in the new show, and of course who will play them. There are actually many factors that can influence the success of “The Blacklist” as a result of its spin-off, and hopefully the showrunners are planning things in a way that they will boost the popularity of both productions.
It was relatively recently that we got the announcement about the spin-off in the first place, so it’s possible that it’s still in a relatively infantile stage and the producers are still trying to figure out exactly how to link it to “The Blacklist” itself.
But that wouldn’t make too much sense in light of the rumors that the new show is already being prepared for a release during the airing of the new season of “The Blacklist”. It’s also possible that the producers have had some ideas about the spin-off for some time before they made the official announcement, and they just had to set things in motion afterwards, following the plans they had already established. In that case, they would have a bit of a head start.