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.
Blog deletion has been discussed before here. Mike agreed with the idea, but was reluctant and limiting blog deletions has not been implemented.
Now I understand that sometimes it may be necessary to delete blogs. In my blog I suggested reaching out to Codeforces administration if that is the case, but that might be too big of a step.
My basic motivation is the same as yours: while the blog is content created by you and should "belong to you", the comments under it aren't and shouldn't. On Codeforces, many comments are at least as valuable, or more valuable than the blog itself. Blog authors should not have the right to delete my comments that I spent a lot of time on. Deleting your blog can destroy valuable information written by other people, and at least for me it is demotivating to write comments at all knowing that they might be deleted at any time. It's not just a theoretical problem, many people like to delete their question once they got an answer, and sometimes blogs containing my comments have been deleted arbitrarily.
Despite my deep dislike of Reddit, I think their deletion model is fairly good. So here is my suggestion. When you "delete" a blog:
This "soft deletion" would always be available to authors. "Hard deletion" — deletion as it is now — would be available for a short time. In fact, such a policy could also be implemented for deleting comments.
This way, we can implement reasonable privacy without deleting other people's hard work.
(To be clear, an individual's comments on a deleted blog aren't technically deleted. They're still accessible on the internet from that individual's comments page, still could be copied elsewhere if they contain important information, still contain the title of the blog post they were posted on for context. The only real difference is that they're in practice not going to be seen nearly as often if the post were deleted, which I think would largely be an equivalent result to hiding the blog from recent actions, perhaps with the sole exception of not killing shared links)
Recently I posted two comments to the blog "Someone stole my code during a contest, possibly using an exploit" which are now completely gone (together with the blog). So what you said is not entirely true. Seems some comments stay while others are removed completely.
The difference is whether blogs get moved to drafts or deleted.
If a blog is moved to drafts, you (but only you) will see them on your comments page. Opening them will show a "You are not allowed to view the requested page" prompt.
If a blog is deleted, the comments aren't visible on your comments page. If you somehow obtain a link to a deleted blog, you will see a "Page not found" prompt.