Page my customers can post to that goes to many wordpress sites

I need one webpage that my customers can go to, to upload a blog post and it will be posted to the 10 or so wordpress blog sites that I am building. I would like the customer to go to a URL and then enter their username and password. From there I want them to be able to add their post with text and images etc. I want them to be able to select from a list of websites that I have predetermined each client is allowed to post to. For example - I may have 10 sites in total but I may only allow client 'A' to post to 8 of them. They in turn, may decide that they only want to select 6 to update. This must be able to be done. When they have selected the sites they would like to post to, they press an upload button. At this point they will be asked if they 'are sure' and that once they have pressed the 'confirm' button it will be live for everyone to see (I will also add a small disclaimer). They will press 'confirm' and the information will be uploaded. All my blog sites will be wordpress sites and under separate domains. They can be on the same server if necessary but I would prefer that that not be. I realise that there are some sites/plugins out there that may already do this sort of thing and I am open to someone just explaining how this can be done, or implementing those for me as the ones I have tried do not seem to work correctly. I only want my clients to be able to blog and not have any other options that would be associated with the backend of a wordpress site. Please provide a brief description of how you intend to do this and the procedure that you will use. It is vital that the blog upload site be easy to navigate and look clean, modern and crisp. I have knowledge in building wordpress sites and understand I may have to load a plugin on the individual blog sites. I may need to build more features into the upload page in future and I am looking for people who can handle the extra work as this project grows. Thank you and Good Luck