I thought the DMs job in 4E was to set up encounters where the monsters get to use their special advantages, and it is the player's job to work around them. Otherwise, whats the point in monsters having situational advantages? Yes, it might have been a bit unfair to put the tram in a locked room with three swarms using darkness and blindsight, but maybe they could pick the lock, put up a wall or fire, or otherwise controlled the situation to their advantage?
While yes, it's true that the DM is supposed to create situations that help out his monsters, he should still put some though into encounter design and how it will impact things.
In the above case it's a confluence of factors that made the encounter rather difficult for the PC's. Swarms are difficult to damage in practice, even moreso when you include darkness in the mix. Having 3 of these monsters in the room was probably too much, and it should've been limited to 1 or 2. This way, they wouldn't be spamming darkness
every round, which actually makes for a more dynamic encounter. With the inclusion of some kind of lurker that could benefit from the darkness, it suddenly becomes kind of scary. Now the PC's will randomly be subjected to a situation where a dangerous lurker could be about to really lay the hurt on them, rather than just saying "Oh great,
another round of darkness where I have a -5 to hit the guys take half damage..."
Locking the room just adds to the issues by taking any sort of tactical maneuvering out of the equation. If the area were more open and you perhaps included some Artillery with an ability to see through the darkness you would force the PC's to think more tactically about the situation. Do they go into the darkness to fight at a disadvantage, or do they stay outside and wait for it to clear? Perhaps they should use some forced movement powers to separate the artillery from the cloud so that they can take them down before dealing with the swarm? More choices on how to act makes for more dynamic, and usually more fun, encounters.
Oh, and as for the Wall of Fire thing, it's never a good idea to count on a
specific PC power to deal with the encounter you made. WoF is a Daily, correct? So what if the Wizard used it earlier in the day and it's no longer available? Or in the case of other powers, what if they miss? It's too risky to rely on the players to use the exact powers at the exact times that you think they should to save an encounter.