I created a blog site and from site settings changed the master page to a custom developed page (having to enable publishing features to be able to do this through the UI). After applying the master page change the new master page did not render on the blog site.
With credit to my colleague Inderjit Singh (Link to Linkedin) the workaround is to apply the custom master page to the system master page section. This master page setting is for all application pages and site settings pages as shown below.
At this point I must stress that applying the master page to the system may cause many knock-on effects and further testing of each application page will be required to satisfy using and suggesting this to a client and could cause a lot more work with regards to styling.
I strongly believe this is not by design behaviour and have raised this with MS and will as always keep you posted.
Further information about the blog template
Each page in the blog site contains a reference to default.master (known as a token) as show below:
Changing this reference to custom.master as shown below will render the custom master page selected in the site master page section (shown in the screen grab above):
The issue of changing this on each page means that each page becomes un-ghosted potentially causing issues later on and i would strongly advise against recommendations in other blogs to edit the pages directly on the file system as this will fix the issue short term but will cause more problems in the long run.
*** Update 18/05/11 ***
After numerous diagnostic tests I was provided with a support KB that was applicable to MOSS 2007 http://support.microsoft.com/kb/936908. The KB suggests two ways to work around the issue. One of these was to simply enable the publishing feature on the site you need to modify the master page (well you have to do this anyway to be able to change the master page through the UI) and this didn't work. The other option was what I described in my original post above which was to change the system.master which I have explained the issues this could cause by doing so.
This case has now been closed with Microsoft on the basis I have been informed that the blog functionality is classed as collaboration (and not publishing) functionality.
After writing various paragraphs here on my view of the above outcome of the case I felt these were best removed!
I welcome your comments and feedback.