Hey. Getting blend modes to work in Spine-Unity involves some extra steps. (Except that we get "additive" for free, because of the way PMA Vertex Colors works)
In 3.6 and below, we required you to create a separate Material that uses a multiply shader for your atlas. Then add the SlotBlendModes component to your Spine GameObject, using that Material as its multiply Material.
In 3.7, you only need to use the Default BlendModeMaterials asset that comes with the runtime, and add it to your SkeletonDataAsset's Skeleton Data Modifiers here:
You can read more about it here: Spine-Unity SkeletonData Modifier Assets
You can find more information on upgrading to 3.7 from 3.6 in the upgrade guide: Spine-Unity 3.6 to 3.7 Upgrade Guide