Don't edit the generated files. Ever. Just. Don't. Do. It.
Instead, make your edits in partial files in a different directory. To add attributes, declare a Metadata class at the top of your partial class definition.
[MetadataType(typeof(BlogMetadata))]
public partial class Blog
{
// it's possible to add logic and non-mapped properties here
}
Now in your Metadata class, you can define attributes or other logic:
public class BlahMetadata
{
[AllowHtml]
public string Content{ get; set; }
}