Resolving Issue with Bing Search Indexing

I acquired a client recently who was using another agency for his website - he wasn't happy with the website and asked me to rebuild it and take over hosting, maintenance, etc.... One of the complaints he had was that his site wasn't indexed on Bing. Historically, it looks like at some point while his website was with the previous agency, it had been SEO-hacked (lots of Japanese results in Google) and supposedly, the agency had cleaned it. I built a brand new site - retained nothing from the old one. Unfortunately, the domain seems blacklisted in Bing. I use the webmaster's console and submitted all the new sitemaps, make sure crawling is allowed - all the standard stuff. I have dozens of sites listed in Bing with no issues. But this one site, just will not index - testing pages with Bing Webmaster shows they are reachable and Bing can read them. So I reached out to Bing support and explained the situation - all I got back was a boilerplate "your site did not meet the standards set by Bing the last time it was crawled. Please review our guidelines to better understand". That's of course zero help and even the console show zero links with guidelines issues. Has anyone here worked through a similar situation and can provide any suggestions?