I found out the reason for my photos looking awful in Blogger. It's down to the new Google+ auto-enhance feature, which is supposed to improve your photos but actually makes them look much, much worse.
The full explanation can be found here: http://www.peekandponder.com/2013/09/how-to-fix-poor-image-quality-on-blogger.html
Simple fix can be found in the Google+ settings where you can switch off the auto-enhance option and revert photos which have already been "enhanced"...you can also set an option to say you want your photos uploaded at full size. Glad I found this now, as I was seriously considering stopping using Blogger altogether.