Think of it as the Das sucked the S out of schwarzes. Funny but works better than trying to remember 48 different combinations for adjective declinations.
You proably meant "schwarzes".
Definite article: weak inflection
But there it says..: Weak declension is used when there is a preceding definite article ("der-word"). And it is 'Das Kleid'.
Thanks! I think I finally got it ;). So it is "das schwarze Kleid" and "ein schwarzes Kleid".
Schwarzes would be the strong ending, but das is already strong, so you use the weak ending, schwarze.
If you think in terms of strong and weak (forget "mixed") this becomes much easier to memorize. If the determiner is strong (i.e. has the der-word endings) then the adjectives after it are weak. If the derminer is weak or absent, then the adjectives are strong.