Hi Nikola,

I had a quick look at your file and found a few little problems.
One important thing to remember with NURBS modelling is clean curves=clean surfaces and bad curves=bad surfaces.

First of all if you zoom up to the area circled in white you will see a small kink/bend in that corner this is causing your gap in the blend surface in that area.



The blend tool will create a surface tangent to other surfaces so that's what is happening with the bottom surface, the blend tool is coming in on it self to be tangent with that bottom edge so you have a situation that looks like the picture below.

You're probably better off using Loft once you've cleaned up the surfaces.



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