Here's my stab at it. The italicized description probably needs to be expanded to include frills, horns, sails, etc.
The dragon has a serpentine body, with thin layers of smooth scales overlapping each other. The scent of damp earth surrounds the dragon.
Slate dragons are benevolent tricksters. They enjoy playing harmless pranks. Slates are the self-appointed protectors of their chosen territories, ensuring that the environment and ecology of the area are not diminished by the actions of other creatures.
A slate wyrmling's scales are blue-gray. As the dragon ages, the scales darken, becoming dark blue or dark purple-gray with streaks of brown. Its old scales rarely drop off; instead a new layer forms over the previous one. Its eyes are indigo with silvery pupils.
Slate dragons lair in cool, rocky areas near large bodies of water, such as coastal cliffs or mountain lakes. Their lairs are usually tight, winding caverns, slick with sea spray or runoff from melting snow. Bronze dragons sometimes share their territory, until they tire of the unceasing pranks and unpredictable actions of the nearby slate dragons. White and slate dragons also cross paths frequently, resulting in occasional conflicts. Whites grow incredibly frustrated by the slate's sidestepping tactics, which counter their preferred method of sudden assaults.
Slate dragons can eat almost anything, including coastal birds and mammals, aquatic and cold-climate plants, and even rock.