I've never focused too much on the actual PageRank on my sites but, after having had a specific one for few years that is still at zero I start to worry that I've been somehow blacklisted. Is there a way to check if that's the case? The site has always been a little slim on content as it's a single-page app.
PageRank zero after a few years
pagerank
Google has stopped publishing PageRank publicly.   The last time the Google Toolbar was updated with new PageRank data was December 2013. Google has said that it will never be updated again.If your site was created after December 2013, your toolbar PageRank will always and forever read 0.  If you changed anything about your URLs since December 2013, your PageRank will be zero.   That includes:Adding are removing the wwwMoving to or from HTTPSAdding or removing index.htmlChanging your domain nameGoogle still does measure and use PageRank internally.   The best way to know if your site has PageRank is to check that it is being crawled and indexed.   If Googlebot is coming back to recrawl your pages periodically, you are in fine shape.