Most visual effects have source and destination blend modes:


Notes:



Source:

from Freshlook:

This is the relationship between the blend modes of visual effects and the D3DBLEND enumeration:

Zero                                     D3DBLEND_ZERO
One                                      D3DBLEND_ONE
Source color                             D3DBLEND_SRCCOLOR
Inverse source color                     D3DBLEND_INVSRCCOLOR
Source alpha                             D3DBLEND_SRCALPHA
Inverse source alpha                     D3DBLEND_INVSRCALPHA
Destination alpha                        D3DBLEND_DESTALPHA
Inverse destination alpha                D3DBLEND_INVDESTALPHA
Destination color                        D3DBLEND_DESTCOLOR
Inverse destination color                D3DBLEND_INVDESTCOLOR
min(source alpha, 1 - destination alpha) D3DBLEND_SRCALPHASAT
Obsolete                                 D3DBLEND_BOTHSRCALPHA
Both blends use inverse source alpha     D3DBLEND_BOTHINVSRCALPHA
Constant blend factor                    D3DBLEND_BLENDFACTOR
Inverted constant blend factor           D3DBLEND_INVBLENDFACTOR
NWN2_BLEND_FORCE_DWORD                   D3DBLEND_FORCE_DWORD

By looking at https://docs.microsoft.com/en-us/windows/win32/direct3d9/d3dblend you can understand the meaning of each blend mode.

SourceBlend seems to be always D3DBLEND_SRCALPHA, no matter the value you set.