Hi,
I do not know where to start and honestly, I have not the time to explain everything in detail. But as you ask me in this very polite way, I will give you some short information and a useful link.
First of all, there are differences between a 'freelancer' (Freiberufler) and a selfdepent (Selbständiger). None of them can apply for a visa/work permit outside Germany. If someone has already a work permit (f.e. a Blue Card) and he is in Germany, he can apply for changing the status from dependent (employee) to selfdependent. Which is not very complecated if one has spend some time in Germany already.
You are talking about a shop!? This is something completely different, because there is no lack of shops in Germany. Remember, the Blue Card is set up for high qualified experts, who are needed in Germany, therefore Germany shows some possibilities to get a permit.
If you want to set up your own company/shop or whatever, Germany wants you to proof that you invest some money and additionally they want to believe that your business concept is of some benefit for Germany.
Here is a link where you can figure out all important information:
http://www.gtai.de/GTAI/
If you want to work as a freelancer in a Blue Card job, you can start with an Umbrella Company (trust7 offers such service) and change to the right status later.
Good luck
Detlef