The advantage about Play store is some paid content which may not be popular and not listed in other stores. Also Play store receives updates on the first place (usually). As Google left business in China, there are hundreds of 3rd party app stores such as hiapk or anzhi etc. most of them has more than 300000 apps but still, all free stuff. There is even a store called wandoujia in Chinese which combines dozens of 3rd party stores into one and has more than a million apps lol. I don't really trust blackmarts as malware can be easily implemented and they do lack some good proper stuff