A familiar scenario — a person chooses to delete a blog and all of the comments are lost. This is quite annoying, since many comments are interesting or funny. I have few ideas that might solve this problem:
Editing and deleting comments will be available for a short period of time (few hours?).
When a blog / comment is deleted, the comments won't be lost. Instead, comments will be hidden similarly to low rated comments. Deleted Blogs won't appear at TOP section or Recent actions. In addition, maybe up votes and down votes will have lower impact.
Those ideas aren't perfect nor complete, I encourage you to offer better solutions.