WordPress.org

Does this plugin modify the database?

No, it does not. So it's safe.

Is blockquote cite link and title visible in the feed too?

Feeds do not load javascripts, so no, you'll not see it in the feed. However, by virtue of the additional information within your blockquotes, title is visible when mouse pointer is hovered over a quote. All credits to you for filling in the info, and none to this plugin for such visibility.