How do I fix blue, hard lines appearing with subdivision modifier?

Question

Before subdivision surface modifier:

After subdivision surface modifier:

Going back to modeling after subdivision surface modifier:

Also, when I add smooth shading, very dark lines appear:

Also, just in case this helps, I stacked mirror modifier before subdivision surface modifier.

Answer 1

you may have right clicked > Shade Smooth by Angle (instead of a simple Shade Smooth) which has created sharp edges over the defined angle (although I'm not sure how it happened here). To remove, switch to Edit mode select all and CtrlE > Clear Sharp.