Any sort of non-toxic plant matter can be used to make black water extract. Basically the black water is caused by tannins that are released from decomposing plant matter. One of the easiest ways is to leave a piece of driftwood in a bucket of water. The tannins will leach out of the wood and make the water go yellow thru to brown, (the darker the colour the more tannins in the water). You use this yellow to brown water for water changes.
You can also use peat from your local nursery or hardware store. Buy a small bag of peat that doesn't have anything like fertiliser added to it. Add a handful of peat to a bucket of water and leave it to soak. Eventually the peat will sink to the bottom and the water will go brown. Again use this water for water changes.
If you want to concentrate the liquid then warm it up so most of the water evaporates. Then pour the liquid thru a fine sieve and bottle the liquid until you require it. Keep the bottle sealed up and airtight.
Black Skirt tetras and most other tetras don't actually need blackwater. If you are trying to breed cardinal or neon tetras then black water can be used. But for general fish keeping, and for fish that are captive bred, you don't need it.