ive got a western pro blade on a unimount. it has a modest amount of rusting on the backside. i see pics of them for sale online that have 3-4 times as much rust as mine. anyway, i believe the rust comes from water/dirt getting trapped between the blade and the rear bracings, specifically in the gaps between the welds. there is a lot of area there for moisture/dirt to collect and stay . i knocked down the rust and painted with POR-15 . i then used rustoleum paprika color paint to paint over it. the paprika is a close match to the faded red on the blade.
finally, i filled in the all gaps with autobody seam sealer, to keep the water/dirt out of those areas. a whole caulking gun size tube was 12 bucks, and i did every seam and still had half left. the seam sealer is paintable, so i hit that with the paprika paint too. i think that will prevent any more rust, and if you have one with no rust and want to keep it that way, the above might just do it.