It's been years since I have used Snort on OpenBSD, so I cannot provide up-to-date knowledge... but from fading memory, it seems that you should be able to analyze logged information from the
pflog(4) device or from
pflogd(8) logfiles.
To log traffic, be sure to add the log option to each filter rule you are interested in, either passed or blocked.
As to why blocked NIC traffic is blocked from Snort? My best guess: Snort is a userland application; PF is part of the kernel. PF prevents userland processes from seeing blocked traffic.