I've been keeping blackwater crypts submersed for decades, often in small breeding tanks and other low-tech set-ups. I never had any problem when their natural environment got reproduced reasonably well:
Rule #1. No carbonates in the water nor in the substrate/mix.
Advice #1. A low pH is your friend.
Suggestion #1. Keep nutrients limiting.
Common sense. Don't overdo lighting.
I'm sure there are dozens of possible ways to do it - I'm relaying just my experiences as some food for thought here...
In my early days, I've usually been using peat to lower the pH (pH 3.5-4.5 is a good target range) of pure rain water (or DI water) and also to add some organic stuff (fulvic acids, etc.). Substrate ranged from pure quartz sand or weathered granite sand to pure peat. Additional nutrients came mostly from fish feeding.
Later on, I experimented a lot to avoid unsustainable use of peat. Adding autumn leaves (beech, oak, maple, etc.) workes quite nicely but you have to account for introducing (also) more readily digestible organic material (most blackwater fishes don't mind unless oxygen levels go down to zero...
).
Nowadays, I usually just use leaf-mold and throw that in a tank filled with DI water. Just kidding? No! This is not for the faint of heart though...
Depending on the quality of your leaf-mold, you have to expect getting some algae but usually the "nasty" ones will get away eventually. You may not want to try this first with a large show tank though.
If you're a bit hesitant, just make an almost fluid leaf-mold slurry and fill the tank 0.5-1 inch (you can use more but it's usually wiser to start with a little and add more later on). Drop in some crypts and let them grow emersed & propagate for a while; also see my thread on emersed culture:
http://www.aquaticplantcentral.com/f...crypts-in.html. (Consider adding some more slurry and waiting some more weeks.) Then
gently fill the tank completely and, presto, you have a real blackwater habitat!
Good luck!
Submersed culture of blackwater crypts 
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