Blend modes
A mesh's blend mode says how its colour mixes with what is drawn under it. Set it in Properties > Blend mode with the mesh selected.
Colour blend
| Normal | paints over what is below |
| Add, Add (Glow) | brightens: highlights, glows, sparkles |
| Multiply | darkens: shadows, blush |
| Darken, Color Burn, Linear Burn | darkening modes as in a paint program |
| Lighten, Screen, Color Dodge | lightening modes as in a paint program |
| Overlay, Soft Light, Hard Light, Linear Light | contrast modes |
| Hue, Color | take the mesh's hue, or its hue and saturation |
| Add (Before 5.3), Multiply (Before 5.3) | the two modes moc3 had before 5.3 |
Everything past Normal, Add and Multiply arrived with Cubism 5.3.
Alpha blend
Properties > Alpha blend sets how the mesh's transparency combines with what is below: Over, Atop, Out, Conjoint or Disjoint. Over is the usual one. The two modes from before 5.3 ignore it, so the row greys out for them.
From your art
PSD, Krita, Clip Studio and cmo3 imports bring each layer's blend mode in, and File > Re-import Artwork… applies changed ones. Vivid Light becomes Linear Light. A mode Ayatsuri doesn't know draws as Normal, and the log says which layer it was.
Blend modes draw on the canvas, in the Render window and in thumbnails.
Exporting
moc3 5.3 stores every blend mode. An older version draws each mesh with the closest of Normal, Add and Multiply, and the export dialog says how many meshes that touched. VTube Studio and OpenVT don't open 5.3 yet, so a model for them should look right with Normal, Add and Multiply alone. See Export.