Anyway, I think several other IJ types are also viewed as 'serious', but I agree that workaholic probably doesn't fit so well with INFJ, as myself and my friends don't really fit that at all.
As for realistic, all of us are quite pragmatic/logical. We're aware of reality, even though we don't like it. This is where the cynical side comes in.
As for dark, I don't think any of us would describe ourselves as 'dark'. However, 3 of us 4 definitely have periods where we are quite cynical and down about things. So we're not really sunshiney people either. :-)