Any SEO idea?

...for many wordpress blogs with mainly the same categories and appropriate content? any trick for improving google visibility?

That's a little vague, as the tricks are going to differ based on the specific config of the blog, how unique the content is, and how "visible" these blogs already are.

In general though, use HTACCESS and Robots.txt to make sure you're limiting the number of URLs that serves duplicate content under any given blog, and look into Wordpress Siloing tutorials or plugins. In short this makes sure your link love is spread around, and that the SEs aren't accessing the same page from a bunch of URLs. Hopefully this prevents them from discounting content on any one domain, so if nothing else at least one blog stays strong.

Altering category names a little to tag long tail, stemming variants, and plurals of your keywords across the various blogs isn't just a good way to help shake up the mix and avoid duplication; it's a great strategy to increase your niche exposure.

And also, don't forget footprinting; try altering names of CSS Classes, and definitely use different templates. You can also rename images, screw around with alt and title attributes for images and links... get creative, code elements count.

well, thanks for your time, i'm asking for a wordpress plugin or a trick that will help me to interlink related words/pics/posts from multiple blogs blogs.

do search in search engine writing word press theme .

Try screwing around with these WP Plugins and see if they suit your needs.