Jigen Sengokushi: Kuro no Shishi - Jinnai-hen

Both OVAs are about ninjas in ancient Japan using mystical powers to fight alien technology. They were also written by Go Nagai (for Black Lion) and Ken Ishikawa (for Miroku), long time best friends that worked together in many series. 

Both titles feature Oda Nobunaga as an antagonist that plans on taking over the world with the help from supernatural beings. Both anime are created around the same period (late 1980's-early 1990's) with similar animation and a similar vibe. The main protagonist in both titles are a young ninja that must protect the people that they value the most and have a past connection with the main antagonist. They are also both action-packed anime with lots of violence and blood. If you like one, you will enjoy the other.  

Ninja Scroll and Black Lion are very similar in both content and execution. Both are set in feudal Japan and feature ninja characters. Both anime are quite action-packed, with a lot of violence and gore, creating a similar tone and mood. Also, the abilities of the main villain in both anime are quite similar.