Concealed seam is like this. （a good balance between looks and strength）
Example 1：seam allowances open
Sew the 2 fabrics with right side together, and press seam allowances open.
Sew on opened seam from the surface.
Fabric and thread, if use the same type of color, seam was rarely unnoticed.
Example 2：bias tape
Finish raw edge with bias tape.
Sew the border line between bias tape and fabric.
Fabric and thread, if use the same type of color, surface seam was rarely unnoticed.
Concealed seam, movie is the following.