You can link a blend curve to a reference plane so Alias always keeps the curve in the plane.

1. Pick the curve and the reference plane.
2. Choose Curves > Blend Curve Toolbox to show an extra floating palette containing the BlendCrv Tools tab.
3. Choose BlendCrv Tools > Blend Curve Planarize.

   The blend curve and reference plane stay linked, so transforming the reference plane modifies the blend curve.

   Note

   To unlink the blend curve from the plane, pick them and choose BlendCrv Tools > Blend Curve Planarize again.