Yes, it means that in the exploit market that kind of exploit can be sold and bought for something in that range (e.g. $25k-$50k). That website doesn't specifically talk about black market:
The ongoing observation of the exploit market structure helps to collect current prices.
but, if you don't report vulnerabilities to bug bounties, the market you'll find is somewhat gray, if not definitely black, as the legality of selling exploits to a third party is dubious.
A well-known company in this sector, Zerodium, has published its payout ranges for different types of exploits: https://zerodium.com/program.html. There is also a famous 2012 Forbes article on the topic.
It is also very likely that vuldb.com crawls black market websites, in order to find more accurate information on the prices. However, there are probably market areas which cannot be easily found, such as the three/four-letter agency market (i.e. worldwide intelligence agencies, such as the NSA). It has been speculated that prices in those areas are much higher than the ones typical of the "standard" black market.