Buildapc

Likely a bios update you need to apply. At one point the B450s weren't going to support Zen 3. AMD reversed course later, but that likely means that a bios update had to come along in order to add the support.

Lots of motherboards, even new ones, still ship with old-ass bios.

Also, remember that memory-training is a thing now -- don't assume that 10 seconds of a black screen means it won't post. Easily wait a minute or more - board could just be in memory-training to get the best speeds.