CatherineIsom said:
this may sound a little obvious, but I am really not sure.
The ISBN that I have been allocated on Smashwords, can I use that on my Amazon page, and can I promote it on my blog OR is it solely for sales through Smashwords?
For registration purposes Smashwords aligns the ISBN to the ePub format of your book - read by Sony, Nook, I-Pad, etc. Amazon uses its own ASIN system so doesn't require an ISBN for a Kindle ebook (AZW format). However, there is, or was last time I uploaded an ebook via Kdp, a space to add in an ISBN. Although it is not used I key in my Smashwords ISBN into the box so as to help tie various versions of the same title together, a sort of belt & braces thinking, in case anyone else uses the title I've chosen.
As to putting the ISBN on your blog... why would you want to? Links to your book's page on major e-tailers would serve prospective readers better. No one has the time to hunt down ISBNs, we all want a one-click system.