Google has added a bullet point to its reviews help document page that says that Google values all reviews, not just positive ones. Here’s what it says:
Value all reviews: Reviews are useful for potential customers when they’re honest and objective. Customers find a mix of positive and negative reviews more trustworthy. You can always respond to a review to show the customers that you care and provide additional context. If the review doesn’t follow our posting guidelines, you can request to remove it.
What does this mean for your SEO?
This should not have an impact on rankings. However, if a business has 99% all positive reviews, Google might flag the profile and potentially remove reviews that they suspect aren’t valid.
Try not to fret too much over the occasional negative review.