Hello,
It looks like the blended surface is performing as expected. I think you’ll always end up with a seam there based upon your input curves and surfaces continuity. One easy way to confirm this is by simply replacing your curves with G2 or higher and see what happens. I think you’ll find those seams will disappear.
On a related note the line work you’re using to create these surfaces is not precise and would cause anyone problems creating surfaces from those. I’d suggest backing up and concentrating on improving your line work and alignment skills, once you have that working better that will act as a proper foundation for surfacing the model. If the construction curves aren’t right your model is going to fall apart at some point, and that can be very frustrating.