Text and description quoted verbatim from Microsoft DirectX 7 help file, included here for convenient reference.

The D3DBLEND enumerated type defines the supported blend mode for the D3DRENDERSTATE_DESTBLEND values in the D3DRENDERSTATETYPE enumerated type. In the member descriptions that follow, the RGBA values of the source and destination are indicated by the subscripts s and d.

typedef enum _D3DBLEND {
D3DBLEND_ZERO = 1,
D3DBLEND_ONE = 2,
D3DBLEND_SRCCOLOR = 3,
D3DBLEND_INVSRCCOLOR = 4,
D3DBLEND_SRCALPHA = 5,
D3DBLEND_INVSRCALPHA = 6,
D3DBLEND_DESTALPHA = 7,
D3DBLEND_INVDESTALPHA = 8,
D3DBLEND_DESTCOLOR = 9,
D3DBLEND_INVDESTCOLOR = 10,
D3DBLEND_SRCALPHASAT = 11,
D3DBLEND_BOTHSRCALPHA = 12,
D3DBLEND_BOTHINVSRCALPHA = 13,
D3DBLEND_FORCE_DWORD = 0x7fffffff,
} D3DBLEND;

D3DBLEND_ZERO
Blend factor is (0, 0, 0, 0).
D3DBLEND_ONE
Blend factor is (1, 1, 1, 1).
D3DBLEND_SRCCOLOR
Blend factor is (Rs, Gs, Bs, As).
D3DBLEND_INVSRCCOLOR
Blend factor is (1-Rs, 1-Gs, 1-Bs, 1-As).
D3DBLEND_SRCALPHA
Blend factor is (As, As, As, As).
D3DBLEND_INVSRCALPHA
Blend factor is (1-As, 1-As, 1-As, 1-As).
D3DBLEND_DESTALPHA
Blend factor is (Ad, Ad, Ad, Ad).
D3DBLEND_INVDESTALPHA
Blend factor is (1-Ad, 1-Ad, 1-Ad, 1-Ad).
D3DBLEND_DESTCOLOR
Blend factor is (Rd, Gd, Bd, Ad).
D3DBLEND_INVDESTCOLOR
Blend factor is (1-Rd, 1-Gd, 1-Bd, 1-Ad).
D3DBLEND_SRCALPHASAT
Blend factor is (f, f, f, 1); f = min(As, 1-Ad).
D3DBLEND_BOTHSRCALPHA
Obsolete. For DirectX 6.0 and later, you can achieve the same affect by setting the source and destination blend factors to D3DBLEND_SRCALPHA and D3DBLEND_INVSRCALPHA in separate calls.
D3DBLEND_BOTHINVSRCALPHA
Source blend factor is (1-As, 1-As, 1-As, 1-As), and destination blend factor is (As, As, As, As); the destination blend selection is overridden. This blend mode is supported only for the D3DRENDERSTATE_SRCBLEND render state.
D3DBLEND_FORCE_DWORD
Forces this enumeration to compile to 32 bits in size. This value is not used.