Story related, can't be missed.
You retrieve the Blue minions from the Empire Sewer area, the first section of this area is relatively straightforward, destroy the houses in your way along the path turning Minion wheels where needs be. You’ll come to a river with a 1-minion switch, a raft floating around and two of the large frogs coming out of a grating. Set your minions in a guard marker () at the raft and wait for it to come around, it’ll crash into the blockage the switch created allowing you to cross to the netherworld gate. Replenish your minions and enter the Arena underworks.
Some Imperial Guard will attack you so take them out and advance for a cutscene showing the Hive. Kill any remaining enemies and smash the barrels on the ramp for some blue lifeforce, backtrack out the way you came and a previously blocked grating will fall allowing more Imperial guard to attack you, kill them and head into where they came from to find a 4-minion wheel which will unlock the grating below. Return your minions to their burrows as they cannot pass through the water and head in yourself.
Follow the path around and kill the two groups of two Imperial Guardsmen (A destruction overlord will have an easier time with this, however making use of your target spell to avoid fighting two at once in melee is advisable for both paths). You’ll come to a 1-minion switch on the left, stand on it yourself to drop the pots held above and release your first blue minion. Return the way you came and you’ll find you can now access (through an open grate near where the first two Imperial Guard were) the area the rest of the pots fell into, bash them open to release 3 more blues and some magical creatures if you feel like killing them. To reach the next area send you broken blues through the grate (one of the blue pots was behind this) and off to the right (opposite direction from the pot) to activate a 4-minion wheel.
In the next area turn right and up the stairs to find another 1-minion switch. Opposite this pots are moving right to leave overhead above two Executioners. Stepping into the switch will drop and pot, time this so that the blue pots drop between the enemies and release and minion to join you. There is a slight delay between standing on the switch and a pot dropping. Don’t worry if you lose a minion as they will keep coming until you have 8, when you do head off to the right to find an 8-minion switch, activate it and return to the upper path to continue on.
The next puzzle has a 1-minion switch and a 6-minion wheel. The switch controls the above claw’s grabbing action whereas the wheel determines the direction and speed it moves in (more minions, more speed to the right, less and it moves to the left). Send six minions down to it ( to select the switch and to send them) and stand on the switch to grab one of the two blues on the left, get off the switch to drop him in the middle. The minion will die so send one of the two remaining blues with you down to revive him with . When the claw is on the right hand side use the switch again to grab one, but this time press to retrieve two of the minions on the switch (leaving 4) and the claw will move left. Again step off the switch to drop a minion and revive him, repeat this for all four minions (and the pots if you want some lifeforce). Aagain don’t worry if a minion somehow dies as they will respawn until you have 12. Notice there is a 20-minion switch in the corner, this leads to a Dark Crystal  but you will have to return for it later. Return back to the path and head right (you’ll receive a cut scene about the “blink” ability of blues leaving them invisible when sweeping.
Activate the 12-minion switch to unlock and area that respawns blues, break open the pots to bring your number to 15. Exit the room and head down left to the next 1-minion switch but activating it does nothing for you, instead sweep your minions through the water, past the Imperial Guards using the ‘blink’ ability and over to the 8-minion switch beside them. Remember to sweep your minions back or they will be visible again and the enemies will attack them. Head over to the now opened area and either sweep the blues past the Imperial Guard and then run past yourself (they’ll ignore you), or kill them. You have unlimited Blues from the nearby spawning area but it will take a long time to kill the guard with them and trying yourself is risky against five.
In any case in the next area fighting is out of the question, use your blues to sweep down and activate both 8-minion switches. One will release a horde of magic creatures on the Empire troops, the other will open the grate. With the soldiers distracted head into the next area. Four Imperial Guard will block your way but, with your help, your blues should be able to handle them in groups of two. Hit the 8-minion switch to allow you to return to other areas but after that return to back the way you came in, there was a 12-minion switch back there which you now have enough minions to press which allows you access to the raft. Select it with and sweep your blues onto it with then step on and hit to activate it. Hold to love forward and to move back. Follow the hive, at one point an Imperial Gaurdsman will hit a switch to block your way, use to dock, kill him and hit the switch to continue on into the Imperial palace, there’s only one path so continue on, looting the chests as you go, for a cutscene.
In the next area free your blue minions and then Enter the Arena for an epic battle, after which you receive your Blue Hive. To deal with the fight see the Boss/Enemy FAQs here: