Indigo Gel is a crafting material that functions identically to normal gel, but makes Indigo Torches instead of normal torches. Indigo Gel can replace normal gel in all recipes requiring it except normal torches.
Indigo Slimes in the Apocalypse event have a 100% chance to drop 1-3 Indigo Gel. This is the only source of Indigo Gel in Terraria, but plenty of Indigo Slimes will spawn in the Apocalypse so it is not difficult to obtain a large amount of Indigo Gel.
Indigo Gel's tooltip also suggests that some sort of berry gives Indigo Slimes their color.
(Edit this page to view the table below properly)
|Indigo Torch (3)||Indigo Gel||By Hand|
|Sticky Glowstick (5)||Indigo Gel|
|Sticky Bomb||Indigo Gel|
|Sticky Grenade (5)||Indigo Gel|
|Lesser Healing Potion (2)||Bottle (2)|| Placed Bottle |
|Indigo Gel (2)|
|Slime Block||Indigo Gel||Solidifier|
|Asphalt Block||Indigo Gel||Blend-O-Matic|
|Stone Block (2)|
- View full table here