There are a couple of multiclass options which might be under rated, IMO.

Both the Druid and Monk use natural weapons. In the Druid's case, natural weapons are gained from Wild Shape, but bear with me. (See what I did there?) And the Monk uses unarmed strikes. Now, we know from the Order of the Lycan that the Crimson Rite ability affects natural weapons. And it lasts up to 8 hours. So....

Depending on how the GM rules, it might be possible for a multiclassed Druid/Blood Hunter to use Crimson Rite on his fists, then Wild Shape into a bear, and have the Crimson Rite still in effect on his claws. Or the GM just might say, No... the Crimson Rite doesn't effect the bear but it returns when the Druid reverts to human form. I don't know what the correct call would be. As to whether this would be OP; the Druid slows down his Wild Shape progression to get Crimson Rite, or the Blood Hunter delays larger Crimson Rite damage for Wild Shape, depending on how many levels of each he takes. I haven't crunched the numbers yet and I probably won't.

The same idea applies to the Monk; he can enhance his unarmed strikes (all of them) with one use of Crimson Rite. In my experience (I'm currently playing a 6th level TWF Blood Hunter) one of the biggest drawbacks of the Crimson Rite is that it lasts for 8 hours, or until your weapon leaves your hand. So keeping one weapon "powered up" leaves only one hand free between fights, unless you want to burn even more HP, and having two weapons with Rites means keeping hold of both them. In actual play, this can be a real pain! The Monk's ability to fight without weapons eliminates this problem entirely; I'd love to try playing a Monk/Blood Hunter sometime. And I'd rate this combo Blue for certain builds. (Shadow Monk with Dark Velocity FTW!)