Making sure that they are in peat moss/ perlite or sand ( silica ) is 1st on the list.
2nd, if you bought something, anything that is bareroot, it will suffer great shock when transplanted, more so if the roots have been handled ( or broken ).
The best things that you can do for the plant is;
1) good lighting
2) keep moist
Sometimes they will take some time b4 they grow out. And like mentioned above, if the roots are messed with ( or bareroot ) the plant will be in shock. So it will just take some time.
The black you are seeing is more than likely the result of the shock that it has been through. It should survive.
If you bought the plants and they were cut back, meaning no leaves/traps on them, it will take a little longer.
I have seen it take 1-2 months to become a full plant.
Also, making sure that you haven't buried the whole plant ( like you would the ones in your yard. You want to make sure that the middle of the plant, where the new leaves come from it not covered with dirt. This will sometimes cause rotting. You want the white fleshy part right below the soil level....not further.
Keep us posted...
Email me if you like:
jaie@petflytrap.com