WARNING!
There is a bug in Rhino 7 and Rhino 8 that makes it impossible to see blend curves during their creation while using some of the custom display modes, including: Shaded 8, Shaded 9, Shaded Grey clay, Bobi X12, Bobi X13, Bobi X14.
On top of that, some other display modes that use Ambient occlusion (Bobi X9, Shaded 5, etc.) are also affected by this bug partially, making the control polygon for the blend curve invisible during its creation. However, the control polygon becomes temporarily visible while dragging the handles to adjust the blend curve. Once the dragging ends, the control polygon get invisible again.
A detailed explanation of this particular bug could be found in the following topic: