Mart,
For the spar, a cylindrical cross-section at the root, changing to an oval, and then to a flattened shape at the tip looks like the optimal design.
It allows the carbon tow to be laid spanwise in straight lines, for maximum bending strenth and minimum twist resistance.
In addition, blade twist causes very little differential length change between the individual tows.
This page has pictures, drawings and calculations on the above.
Dave