The blends you add aren't actually new items. Instead, blend type is stored in a data component.
Warning
Recipes for inexistent blends will still work and display in recipe viewers. By default, these will use a bland blend texture.
Simple Shapeless Blend Recipe
Blend recipes use a components object to set biomeblends:blend_type to the right blend type.
The following recipe uses a bland blend and a dirt block and creates a plains blend as a result.
{"type": "minecraft:crafting_shapeless","category": "misc","ingredients": [{"item": "minecraft:dirt"},{"item": "biomeblends:biome_blend"}],"result": {"components": {"biomeblends:blend_type": "minecraft:plains"},"count": 1,"id": "biomeblends:biome_blend"}}