Green Blood

Green Blood is a man-made substance made using the Lineage Factor of Devil Fruit users in order to mimic the fruit's power. It was developed by Vegapunk, and used in his Seraphim project to mimic several Paramecia powers.

History
While trying to manufacture artificial Devil Fruits, Vegapunk discovered that each Devil Fruit type has a unique way of being copied, with the Paramecia type requiring a sample of the Lineage Factor of a user whose Devil Fruit power one wishes to mimic. That sample is synthesized and used to create the Green Blood.

Vegapunk used this to mimic several Paramecia powers and used them in the Seraphim project.

Trivia

 * While it is confirmed by Guernica that Vegapunk used Kaidou's Lineage Factor in order to create Momonosuke's Artificial Devil Fruit, it is not said how it was used, and it was shown the Seraphim need to be connected to Green Blood in order to use each Devil Fruit power, and not that a new Devil fruit is made available for consumption.
 * Interestingly, SAD, another substance used to create artificial Devil Fruit, in this case SMILE, is also colored green in the anime and Digitally Colored Manga.