The use of "so" in that sentence would be correct...it expresses that the adjective is noted to a great degree. Similar to "чрезвычайно". The use of "so" as a means of comparison is improper. There you're mistaking the word "so" for the word "as". To write that sentence, one would say,"That blue dress is not as elegant as the red dress." It's a common mistake, really. We have a term called "rednecks" which basically means "uneducated". One sounds "redneck" to say "That dress is not so elegant as..."...lol.