Is PBN is black hat method for SEO?

Yes, PBN is black hat, the blackest. Google absolutely hates these and will immediately penalize your site if they catch you using them. This can be devastating, as they tend to be pretty expensive to buy pre-made.

They are not safe, but they are highly effective and can jettison your rank up in the SERPs until you're caught.

If you have confidence in the network of PBNs you've chosen, they will rank your site. But, if Google catches you, you're sites going to take a hit.

PBN stands for Private Blog Networks and yes, it is black hat if you take Google's word for it. the term "black hat" really boils down to whether you comply with Google's philosophy regarding link generation on the internet. According to Google, the only legitimate way websites can generate backlinks is when they earn it through their content.