In Riders of Berk, Mildew's Misery looks like an ordinary Scauldron, being sea green. Rise of Berk however states that eating too much Blue Oleander causes a Scauldron's skin to change color to sea blue.
In the episode Dragon Flower, the dragons of Berk are getting sick because of the Blue Oleander Mildew planted. Fishlegs states that the only cure is Scauldron venom, so they go search for one, luring it with Blue Oleander. Mildew's Misery appears and fights with the vikings. He bites Mildew's butt to get a flower and then swims back in the sea. Gobber manages to extract the venom from Mildew and saves the dragons.
His name was first revealed in this game along with the fact that Blue Oleander causes color changing to Scauldrons.
Mildew's Misery got it's name for the literal misery it brought to Mildew as it bit his butt, infecting it with venom, because there was a Blue Oleander stuck around the area after a whole bunch of flowers fell on him earlier.
The source of the blue skin color was discussed in Rise of Berk's Facebook page.