Blood does two things badly: Mobility (lol DK) and AoE threat/dps.
Every other aspect; On-demand mitigation, sustained mitigation/healing, cooldown availability and effectiveness, burst threat, single target dps, mob control, kiting, even dealing with M+ affixes, is excellent.
Every class/spec needs to have a weakness. It just happens that DKs is mobility, but you still don't get everything. Prot Warriors have been in a bad place ever since the Ignore Pain nerf through the floor. They have the advantage of Damage. Not, you know, the more important part of their actual job.