According to EDHREC, a great resource for seeing how much a card is played in the EDH / Commander format, Contamination is played four times as often as Infernal Darkness - about 400 to 100 out of a similar ratio of decks. Therefore, the Urza’s Saga Enchantment is pushing $9 (and rising) while Infernal Darkness languished well under $1 for a long time.
Is Infernal Darkness more popular than EDHREC considers it to be? It’s certainly never going to be a bulk rare again, simply from the price benefiting from very old set’s supply drying up. Don't rush into buying this card if you don't need it, though. But if you have any around, I’d definitely look to hold them long-term. Contamination is strictly better, as the cumulative upkeep of Infernal Darkness can add up rather quickly. But in the only formats that Infernal Darkness would ever see play in, which are Commander and "kitchen table" casual Magic, having another mana denial card in a Mono-Black control deck’s arsenal never hurts. It can be a useful redundancy and complement to what could become an extremely rare card.
While not on the Reserved List, it's doubtful that Infernal Darkness will be reprinted due to the Cumulative Upkeep requirement. Contamination is more likely to see reprint. But like Infernal Darkness, forcing all lands to tap for nothing but Black mana doesn't seem like something Wizards of the Coast really wants to bring back. This is a long-term investment that could hit or even pass $5 eventually if it's not reprinted. Just be careful that it's not reprinted, or its value will become bulk almost immediately.