Sharepoint: Where to store assets used in the custom master page

Yes the blog sites use a different masterpage. You will have to go into the blog site using SharePoint designer and create a custom master page in the gallery.

I heartily recommend using the CKS:EBE if you'll be using blogs on SharePoint. It has a multitude of improvements over the out of the box blog template including theming, trackbacks, spam protection and many others. Give it a shot!

There is nothing like SharePoint ContentPlaceHolders & non-SharePoint ContentPlaceHolders. When you add a Default content to the Master Page ContentPlaceholder and if you dont override it in the Content Page then it will be visible in all the Pages that use that Master Page. Only way to get rid of it is to Override those Content Place Holder with empty content in the Content Pages. As overriding contentPlaceHolder in all the Pages is not an good Aproach. What I recommend is not to place conent in the ContentPlaceHolder of the Master Page but instead , put the content in the Content Control page of the Pages, so that Content doesn't come in all page but only the page that needs it. Yes it not a good to put the exact same content in multiple page. but in your case that is only the Option. May be you try to wrap the content in to a User Control so that the code can be updated only one place