What black hat SEO techniques still work and stay undetected by Google?
Black-hat SEO is something nobody should do, and most techniques such as duplicate content don't work thanks to Google's updates over the years. On the other hand, there are certain techniques that people use, which are extremely hard or even impossible to detect.
Link Buying
While buying spammy links to irrelevant websites with no authority is a useless black-hat technique, people can buy links that appear to be authentic and place them on industry-related websites that are reputable. Doing the latter is nearly impossible for Google to detect, because the links don't look like spam and they're appearing on authoritative websites. Still, you're better off practicing natural link building if you really want to get the best results.
Link Networks
Many experienced black-hat SEOs will build vast networks of websites to help them rank. The ones who experience the most success create sites that look legitimate and mostly are. Google may struggle to detect illegitimate sites, and links connecting them all to the site trying to rank may also appear natural. It's still a better idea to stick with creating legitimate websites that help generate good traffic with high-quality content, as a lower bounce rate can help you rank and get customers.
Scraped Content
Certain attempts at scraping content from other websites can land you in trouble with Google results, but there are some exceptions. For example, a website might rip content from an infographic that's an image or .pdf, post it on their website as a blog post, and outrank the infographic's creators. Without certain tags, Google can struggle identifying the original over copies, but it's still as tricky as it is unethical to do.
Auto-Generated Content
Most content that's auto-generated looks awful. Robots will take existing content that's readable, use out-of-context synonyms in the place of certain words, and put it all together in a mess that nobody will read. However, there currently isn't a way to make good and fresh auto-generated content, but someday this might change as AI continues to develop—we've already almost seen an auto-generated movie trailer (a human still had to edit the AI's chosen film sequences).
Ultimately, you should still practice white-hat SEO. It's guaranteed to give you much better results than black-hat attempts today, and it makes you much more reputable among the SEO community.