Blissey will be involved in the next Pokemon Scarlet and Violet Tera Raid, which starts on March 24, 2023. This will provide a chance to acquire additional Tera Shards and Exp. items.
The Pokemon Scarlet and Violet Blissey Tera Raids will be five-star. This means that people won't have to complete the game or post-game activities to access them. However, the announcement does mention "bonus Exp. candies."
The second part of the Decidueye Tera Raid will take place March 24-27, 2023. (The first took place March 17-20, 2023.) That was a seven-star one with a flying Tera mark.
The Blissey Tera Raids will be held March 24-26 and 2023 on Nintendo Switch.