Mildew's Misery is a Scauldron which appeared in the episode "Dragon Flower" in Riders of Berk. It's name was first revealed in Dragons: Rise of Berk. 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 (for more information click here). As it made it's way into Rise of Berk, a new discovery was made. A Scauldron eating too many Blue Oleanders at a time may cause their sea green skin color to change to a sea blue skin color. This is because the pigments of the blue pollen can cause many different effects to the Scauldron's internal body, as they are poisonous to almost all reptiles, except Scauldrons.
The source of the blue skin color was discussed in Rise of Berk's Facebook page.