Building CyanogenMod 14.1 for Nexus 6
You can't build that from the official repos yet, since the code is not ready.

There is a reason unofficial builds exist - the devs write their own codes and patches to make the code buildable, bootable and functional ahead of release, and put them in their own repos. Unless you're skilled enough to do the same, you do have to use their code or wait for official nightlies to drop (by then the official code will be ready too).

As for the trust issue... Devs don't usually interfere with CM common code; rather they create device, kernel and vendor repos which are device-specific. Again, if you still can't trust their knowledge about the device then it's up to you to research from the ground up.

Finally, since CM12.x, the blob extraction step is obsolete.

I build CM on my own PC too, but I sync from third-party repos when possible (e.g. if another dev posts his CM14 builds, I get the repo from his XDA thread) and trust them, because why not? CM is made up of individual maintainers; if you can trust one there's no reason you can't trust another.