Don't think this is a Trump thing - as it has its roots back in 2014. Likely what's in this has been carried forward by the FBI since then.
Here is an FBI document from 2017.
Black Identity Extremists Likely Motivated To Target Law Enforcement Officers
I do agree that as of late, this activity has seemed to subside and is not as big of a deal as in the past. But this is not something that racist Trump has brought forth because he hates black people. This is about FBI assessments from years ago.