The Legend of Zelda: Tears of the Kingdom features a wide variety of collectible materials that Link can use for everything from crafting elixirs to upgrading armor. 
Blue Nightshade is a glowing flower that can be found growing in multiple regions throughout Hyrule, atop hills and near small bodies of water.
Blue Nightshade can be used as a cooking ingredient to make stealth-boosting dishes, for creating dye at the Kochi Dye Shop, and in upgrading the Stealth Armor set. Here are the best locations for farming Blue Nightshade in Tears of the Kingdom.
Rutile Lake
One of the best places to farm Blue Nightshade is Rutile Lake, specifically at coordinates (-2472, -0650, 0207), located southwest of Satori Mountain. This lake and its surrounding area are not only a fantastic place to find 7-9 Blue Nightshade, but the player can also collect many Silent Princess and other valuable plants including Endura Carrot and Fleet Lotus Seeds. The area is also rich in wildlife, allowing Link to collect a wide variety of useful ingredients including Chillfin Trout, Sneaky River Snails, and Raw Bird Drumsticks from wild fowl.
East Hill
East Hill is located at (coordinates 1870, -0821, 0237), just north of Kakariko Village in West Necluda. The hill’s summit is an excellent spot to collect numerous Blue Nightshade in a small area. Here the player can collect anywhere from 10-14 Blue Nightshade and a handful of Silent Princess. The quickest way to get to this location is to fast travel to the Makasura Shrine and travel north through the village and up the hill, through a combination of climbing and use of the Ascend ability.
Great Hyrule Forest
The player can find Blue Nightshade deep in the Great Hyrule Forest by heading northwest from the Korok Woods at the forest’s center. However, to enter the forest and reach this area, the player must first make it through the Lost Woods. Unlike in Breath of the Wild, in order to reach the Korok Woods in Tears of the Kingdom, the player must first head underground into the Depths. Follow this detailed guide to reach the Korok Woods and collect its bounty of 7-10 Blue Nightshade.
Pond in Farosh Hills
The player can find a small pond at coordinates (0607, -2563, 0074), in the Farosh Hills area of the West Necluda region. This location can be reached by fast traveling to the Popla Foothills Skyview Tower, launching into the sky, and gliding south toward the small lake below the “Farosh Hills” text on the world map. The player can typically collect 6-7 Blue Nightshade at this location, and also snag a few Sneaky River Snails while there.
The Legend of Zelda: Tears of the Kingdom is available now on Nintendo Switch.