Notes native encryption is not supported for Domino 8.5.
Refer to KB05489
for a list of pre-requisites for IBM Lotus Notes native encryption. See the listed supported Domino & Notes version in it:
Link: View Document
Also, BES install guide for 4.1 sp6 stated:- native notes encryption
The BlackBerry Enterprise Server supports native notes encryption with IBM Lotus Domino server version 8.0.1.
Apparently the support was added later to 8.0.2 too. But for now 8.5 not supported.