To the best of my knowledge: no, there are no lightsaber combat forms specific to the dark side, either named or unnamed. Of course, each lightsaber combatant has his own distinctive style which may be a mixture of several different forms. Dark side practitioners generally have a more aggressive style but no source I'm aware of treats any of these styles has a complete lightsaber form.
Dark side practitioners tend to use one of the seven lightsaber forms, of which Juyo/Vaapad is the seventh. For example, Tyranus (a.k.a. Dooku) was a well known user of Form II Makashi, though that form was also used by Jedi such as Ki-Adi Mundi. Similarly, the aggressive Form VII Juyo was used by both light and dark side practitioners: Legends sources state that various Sith and Dark Jedi (including Darth Bane, Darth Maul, and Darth Vader) used it, but it was also used by Jedi Masters such as Vrook Lamar, and the Jedi Master Mace Windu created the Vaapad variant of the form.
Many Sith and Dark Jedi are former Jedi who learned lightsaber combat as a Jedi and may have simply continued to use the techniques they'd learned while aligned with the light. However, the fact that these dark side practitioners continued to use the seven "Jedi" forms rather than inventing their own "Sith" or "dark side" forms suggests that the lightsaber forms are not specific to the light or dark side. The lightsaber forms are associated with the Jedi mainly because the Jedi seem to have invented them.
There are also some techniques, such as Jar'Kai, which are not one of the lightsaber forms but are nonetheless distinctive. These techniques are also used by both light and dark side practitioners -- Jar'Kai, for example, was used by both the light sided Ahsoka Tano and the dark sided Asajj Ventress.