Either the Tangent setting should work without Interior shapes being selected (preferred).
- or -
The Tangent setting should only be available if Interior shapes is selected.
Looks like this thread: BlendSrf in Rhino 6 different
A post by @pascal from 2020: Blendsrf issue V6 vs V5 - #5 by pascal
Again, this is currently a bug because BlendSrf does not do what it claims to unless an apparently un-related option is selected.