BlendCrv Tools > Blend Curve Planarize

Locks or unlocks a blend curve to a reference plane.

Pick a blend curve and a reference plane, then choose this tool to lock the blend curve to the reference plane.

Pick the blend curve and choose the tool again to unlock the curve from the plane.

Notes

If you do not pick a reference plane, the tool creates one.
If the blend curve is attached at its ends, the plane is created from those two points and the view direction.
If you pick blend points (constraints) instead of the entire blend curve, the plane is created from the locations of the blend points.