Thanks for contacting "Ask Extension" with your question about cardboard mulching.  I found no research that would indicate that toxin's from cardboard used as a weed suppressor would taint the soil or plants. If you are concerned you could remove the corner sections of the cardboard that were glued together and avoid using cardboard that has colored ink other than black.

I did find an interesting article from the University of Washington (The Myth of Paper-Based Sheet Mulch) that concluded that newspaper and cardboard sheet mulches can be effective for annual beds if they are properly maintained. 
But are not appropriate in permanent, ornamental landscapes or non-maintained sites. The article maintained that newspaper and cardboard sheet mulches were often not as effective as other organic mulches (e.g. wood chips or bark) in preventing weed growth or improving yield.
  Here's a link to the full article: https://s3.wp.wsu.edu/uploads/sites/403/2015/03/newspaper-mulch.pdf
Happy Gardening!

Thanks Judith! You're very welcome!
Deb