That is what you want and what the Betta likes....the longer it steeps the more tannins will be released the darker the water....however, make sure you have the replacement water for water changes tanned with the IAL too...especially with 100% water changes...with partials its a good idea too....but not as critical...depending on the amount of tannins and pH changes......
Fresh active carbon will remove the tannins and so if you are using carbon in the filter you may want to take that out.....
Are you using the IAL for treatment or to keep your Betta in tanned water.......