Is Duolingo teaching us correctly? I go around other learning sites to see what they offer in French. One of the websites I liked was Babble. Except it formed the sentences differently. Instead of Elle a bleu robe. They would say Elle a robe bleu with the adjective after the noun. I'm just wondering which way is correct or if it doesn't even matter at all. Thanks in advance.
Some adjectives go before the noun they modify, some after. Here on Duolingo (when it comes to colours, like for the example you wrote) you would write "Elle a une robe bleue", much like you would see on the site you mentioned.
The position of every adjective matters indeed, take a look at these four pages for some clarifications: http://french.about.com/od/grammar/a/adjectives.htm (fourth page in particular talks about positioning)
No problem, if you need some more I suggest bookmarking that site, as there's pretty much everything you would want/need when it comes to the learning of French. At least so far, it was always able to answer my questions.
There is no "one" teaching method, DL is not an online institute but if you really study each lesson and look for some grammar explanations it is more than enough to start a language. Remember there is a "discussion" tab in each exercise where you can ask anyone what you don't understand. Oh yes, and you have to pay for Babbel! ;)