AS by its name SharePoint says it all, Share everything at a single Point to stop duplication of data, So If everything related to branding can be shared within Site Collection, I am not an expert but this is my Opinion, and you can create just one site definition and another Site template with a custom master page which can be shared between site definitions to create thousands of blog sites.

One thing out of topic I think yu should consider is that, if you will create blogs in SharePoint which means it will be doing some Application pool recycling at midnight or some other time on and blogs wont gonna be accessible (I am not 100% sure if it happens everywhere but it does happens in our SharePoint application)

Also another thing you will be considering is Memory Management, If there are thousands of sites, every single line of your code for webparts must be following SharePoint best practices.

AS by its name SharePoint says it all, Share everything at a single Point to stop duplication of data, So If everything related to branding can be shared within Site Collection, I am not an expert but this is my Opinion, and you can create just one site definition and another Site template with a custom master page which can be shared between site definitions to create thousands of blog sites.

One thing out of topic I think yu should consider is that, if you will create blogs in SharePoint which means it will be doing some Application pool recycling at midnight or some other time on and blogs wont gonna be accessible (I am not 100% sure if it happens everywhere but it does happens in our SharePoint application)

Also another thing you will be considering is Memory Management, If there are thousands of sites, every single line of your code for webparts must be following SharePoint best practices.

btw If i was you, I would contact some SharePoint Hosting companies and get some advice as they must have done something similar, I know this company that does help SharePoint user group and other people, you must give them a try :)

this is a sharepoint hosting company that might provide you free sharepoint blog only if you request

No I am not a marketing guy for them, I just seen a MVP telling about them here

SharePoint blogger with extra ordinary architecture knowledge

AS by its name SharePoint says it all, Share everything at a single Point to stop duplication of data, So If everything related to branding can be shared within Site Collection, I am not an expert but this is my Opinion, and you can create just one site definition and another Site template with a custom master page which can be shared between site definitions to create thousands of blog sites.

AS by its name SharePoint says it all, Share everything at a single Point to stop duplication of data, So If everything related to branding can be shared within Site Collection, I am not an expert but this is my Opinion, and you can create just one site definition and another Site template with a custom master page which can be shared between site definitions to create thousands of blog sites.

One thing out of topic I think yu should consider is that, if you will create blogs in SharePoint which means it will be doing some Application pool recycling at midnight or some other time on and blogs wont gonna be accessible (I am not 100% sure if it happens everywhere but it does happens in our SharePoint application)

Also another thing you will be considering is Memory Management, If there are thousands of sites, every single line of your code for webparts must be following SharePoint best practices.