The term "blend mode" is a term of art in the industry, yes. But "mode" is always redundant - lots of properties switch "modes", particularly the ones with keyword values. For example, we could have had "display-mode" rather than "display", but that would definitely have been redundant. The word "blend" isn't used for anything else reasonable in CSS's wheelhouse, so using it by itself would have been fine.
I'm not sure what difference you think you're alluding to between RGB and RGBA. All browsers store colors as RGBA 4-tuples. Are you perhaps under the impression that "RGB" and "RGBA" refer to particular color spaces or something? They don't - nearly all color spaces are alpha-agnostic and can have an alpha value appended to their color definition. (The few exceptions are things like pre-multiplied RGBA, where the RGB components depend on the alpha. But these are rare)
Given that the CSSWG's membership is largely browser implementors, I'm pretty sure we have a handle on what the fuck our graphics stack does.