I believe the blend amount increases by 1.f/(float)BlendDepth for each bone up to a max of 1.f, so using a value of 2 will give you 1 → 0.5, or 3 will be 1 → 0.6 → 0.3 etc.
Look at line 1111 in Engine/Source/Runtime/Engine/Private/Animation/AnimationRuntime.cpp - this is the only place it seems to actually be used apart from its declaration in the struct.