The 5% main stat can actually be made up for on one single item (especially if you are talking about chest/head/pants). My Hunter is rocking leather and mail and has 13592 agility. if I took away the leather chest, shoulder, and wrists, and went back to my 715ish ilvl mail pieces for the 5%, he'd be below 12000 agility (with the bonus 5%) and he'd have substantially lower secondaries. Those leather pieces aren't even particularly high ilvl (850 shoulder, 840 chest, 835 wrists with no tertiary stats). If I went to higher ilvl pieces or pieces with leech or sockets, I could get even more ridiculously OP. My unbuffed secondaries with mixed pieces: 49% crit/haste, 79% mastery, 18% leech, 21% vers. Those would be MUCH lower with only mail. This toon wipes the floor with Legion dungeons and doesn't have much trouble with 110 rares in Suramar. It's also been slaughtering lesser geared 110's and absolutely wipes the floor with leveling toons below 106.
That said, I am not sure about Blood DK (or any strength toons) mixing gear because you lose out on the strength. Even if the secondaries shoot up (which they will) there is no way to make up for the strength. If you do it, you'd want to do at least a couple of pieces, and you'd want them to be the highest ilvl possible, and/or have sockets and preferably leech to make up for the lost strength. That would give you a toon that is virtually unkillable, but the downside would be it would take you forever to kill stuff. My suggestion, if gold isn't really an issue, get a couple of pieces: leather chest and mail belt, since they tend to be the cheapest and they still have decent armor, and try it out. If you keep your eyes on the AH, it is easy to find those two for 20k each or less at 835, and there are often pieces with sockets for the same price. You might find that sometimes you want to kill quicker, but other times you want the survivability. That is what I plan on doing with my plate toon when I make it.