Hi again,
I have needed to extract selected blobs as separate surfaces (and also separate volumes) recently and found a way to do it within ChimeraX version 1.3 (2021-12-08), here's how:
1. Right mouse → Blob → right click desired blob to color it (any color different from the rest of the volume)
2. Split the surface based on colors:
sop splitbycolor #1.2
3. Use the desired split surface to mask the original volume data:
vol mask #1 surf #2.2 fullMap true
4. Save as a new volume:
save my_segmented_piece.mrc #3
Hope this helps :-)
Best wishes,