arp(8) is still supposed to honour the "blackhole" and "reject" keywords with the -S option but it doesn't work as described in arp(8). I have tested and both: 13.0-STABLE and 11.4-STABLE seem to be affected. If this keyword is deprecated then arp(8) manpage should be updated to reflect this change, otherwise, it looks broken.
Raised https://reviews.freebsd.org/D32240 to support the functionality on the kernel side.
(In reply to Alexander V. Chernikov from comment #1) The patch worked for the packets of local origin, forwarded packets weren't blackholed if I recall.