I have modified a hair nif mesh in Blender and then I have exported it. Next, I've made all that block copy/past branch stuff, my custom nif has textures and is clearly seen in Nifskope, but in Geck, when I am trying to add this mesh, it's always invisible, as well as it is in-game. I've read that the trouble might be with Shader Flags, but I've tried saving it with SF_Empty and the original mesh-like ones, both copies have the same problem. How can I make it visible?

Export settings:

Make sure that the hair's primary Nitrishape is named 'NoHat'.  Also, hair has to have egm/tri files.  That's what tells the hair how to change shape, should you modify the shape of the head itself in GECK/character creation.

Yes, it's working! The solution was so simple, lol. Thank you, Tes.

No worries, glad you fixed it.