I believe that blake8086 was referring to the content of the hidden message when he said plain text. He is asserting that if you hide 'hidden message here' in something, you are using security by obscurity, but if you try to hide ENC('hidden message here', 'secret key goes here') you are going to make the detection of the presence of your secret message easier.
I argued against the latter point: if you hide the encrypted message in something which is normally compressed such as JPEG or DivX, the encrypted message blends in with the rest of the data because compressed data has high entropy (the better the compression, the higher the entropy). I don't get your point about security by obscurity, that's pretty much the whole point of steganography I would suppose. Security by obscurity is mostly a slogan to criticize not publishing algorithms etc.
You need to quantify "blends in". If I [an attacker] plot a distribution of the entropy in all your files, and some of them are outliers, even by a small amount, I can focus all my analysis on those files.
