Monster Blob is one of the two skins released in the Season 12 (Halloween) event. It could be acquired for a limited time from mid October to early November 2020.
Monster Blob could be acquired by reaching Rank 3 and by purchasing a Golden Pass for $4.99 during the event period. There may be few opportunities to acquire this skin in the future.
Details of this skin may vary depending on location, platform or time.