Normal
The trash in Blood Furnace is made up largely of Humanoids and Demons. There are a few large pulls, but most are within normal range. Having a Warlock in your group makes things easier as they can Banish/Enslave demons when necessary. Mages and Rogues are also very effective crowd control. Some of the fights run long, so a Shadow Priest can be of great help. Having a Mage or Druid around to remove curses on the fly is also incredibly helpful.
Throughout the instance you must beware of Orc Rogues stealthing around. These Rogues have enhanced AI, so that they ignore plate wearers and go straight for clothies - especially healers. There are several on and around the staircase near the entrance, and in a few other dark corners. You should have someone scout the area before making major pulls.
There are several large rooms full of packs of mobs, as well as roaming patrols. These pulls must be made with patience and care, to avoid pulling multiple groups. You must also watch out for Shadowmoon Technicians, and kill them quickly in any pack their in. They will silence the group as well as throw bombs and drop landmines, making them extremely deadly.
Heroic
Trash isn't changed on heroic mode besides the normal damage increase. Crowd control is essential as with all heroics, but this trash isn't particularly difficult.
