The converted mana cost for Nettlevine Blight is six. You will need two black mana and four colorless mana to play Nettlevine Blight. For what Nettlevine Blight can do, the converted mana cost is well worth it. This is a card that is meant to be played midway through the game. Keep in mind that Nettlevine Blight is an enchantment. It simply means that Nettlevine Blight is susceptible to anything and everything that affects enchantment spells.
What does Nettlevine Blight do in the first place?
When Nettlevine Blight is cast on the battlefield, you enchant an opponent's creature or land. Make sure your opponent has no means to destroy that enchantment. At the end of that player's turn, s/he has to sacrifice whatever creature or land is enchanted with Nettlevine Blight. Then, that player has to attach Nettlevine Blight to another creature or land s/he controls. Systematically, you are dealing away with an opponent's ability to produce creatures let alone defend with them.
If you have a few Nettlevine Blight cards on the battlefield, you can really mess up an opponent's game. In a free-for-all match, four Nettlevine Blight cards will really change the dynamic of the game.
In my case, I used Nettlevine Blight in a pseudo-Grixis EDH (Elder Dragon Highlander) deck that uses black, red, and blue cards. This is due to being able to combine the ability of Nettlevine Blight with the abilities of other cards in my EDH deck.
In that EDH, I used "Thraximundar" for my general. Whenever Thraximundar attacks, the defending player has to sacrifice a creature. Thraximundar gets a +1/+1 counter on it whenever a creature is sacrificed. Creatures simply need to be sacrificed. With Nettlevine Blight, I can force an opponent to sacrifice his/her creatures. In turn, I can put +1/+1 counters on Thraximundar.
The ability of Nettlevine Blight can also be combined via "Prince of Thralls." Prince of Thralls allows any permanent belonging to an opponent to return to the battlefield under your control if sent to the graveyard from the battlefield unless the owner pays three points of life.
Overall, Nettlevine Blight is a pretty good enchantment card for any deck that is using black cards. When combined with the abilities of other cards, Nettlevine Blight becomes a pretty good card to use.