Use Sewing Machine To Attach Extension To Skirt Without Thread Appearing On Front

blind hem with hand is the right way, You can blind hem with machine as well but that needs practice.
I just attached once to my a dress, and just sewed it on, did not do blind hem. You case is better, because
it is dark skirt and full length. So even if you machine stitch, wont be that visible.
You can also sandwich your ruffle edge between skirt and bias tape or facing to look neater.
Other way is - do you have lining, attach it to lining and tack it as few places like on seams ( like indian lehangas)
Sorry I do not know much. But I remember same confusion I had long back. Use to search for videos on how to attach ruffle trim to hem. I did not know much that time so my concern was ruffle edges should be hidden from inside also and fully covered. I did not bother much about outside which is funny ( because i was still learning and did not know much )