Start by making sure you have iBooks installed on your iPad or iPhone.
With your iPad or iPhone in hand:
In Safari go to Blurb.com and sign in to your account.
Under My Projects & Account go to My Book Projects.
Find the icon for the ebook that you want to open. If you have purchased the ebook already you will see a button that says "Download (send link)". Click that to download it. (If you haven't yet ordered it, do so now and download the file).
After the file downloads choose "Open in iBooks" to open the file. Getting an error message? Make sure you have iBooks (available in the App Store) first!