Update: Blogger has decided not to change their policy after all, so it looks like there's no worries for those of us with the "adult content" notification.
I received the following message from The Blogger Team yesterday. The last time we got a notifcation like this, I know many bloggers didn't receive it, so I thought I'd post it here for those who may have missed it.
Dear Blogger User,
We're writing to tell you about an upcoming change to the Blogger Content Policy that may affect your account.
In the coming weeks, we'll no longer allow blogs that contain sexually explicit or graphic nude images or video. We'll still allow nudity presented in artistic, educational, documentary, or scientific contexts, or where there are other substantial benefits to the public from not taking action on the content.
The new policy will go into effect on the 23 of March, 2015. After this policy goes into effect, Google will restrict access to any blog identified as being in violation of our revised policy. No content will be deleted, but only blog authors and those with whom they have expressly shared the blog will be able to see the content we've made private.
Our records indicate that your account may be affected by this policy change. Please refrain from creating new content that would violate this policy. Also, we ask that you make any necessary changes to your existing blog to comply as soon as possible, so that you won't experience any interruptions in service. You may also choose to create an archive of your content via Google Takeout (https://www.google.com/settings/takeout/custom/blogger).
For more information, please read here (https://support.google.com/blogger?p=policy_update).
The Blogger Team