Hi Burr, yeah it is that center part that is the problem. Just a regular Blend is not quite good enough for that area, because Blend makes a smooth connection between 2 different sides, but in that area you want a smooth connection between 4 sides, not just 2.

You can fill the hole with something like Network or Sweep, but it will have a crease there instead of it being a smooth fill.

If you can make the hole to be a pretty small size then the crease may become difficult to see though.

But yes, filling in holes like that with a smooth surface is what an N-Sided patch tool would be used for, there is not really another tool that does the exact same job.

- Michael