You can go with Cryptocurrency market. Go figure out the market and publish daily updates or market tutorials.
People are going to buy more cryptocurrency in upcoming future so you should know what app are related to crypto and help you to get good deals.
There are a lot of customers who want to buy or invest in crypto, so you can start a blog on how to do research from beginning to advence.
Blog on Cryptocurrency trends is one of the best ideas to start a blog if you can provide your user daily uptadates.
People are wondering for earning bitcoins as fast as possible so you can guide them to do so.
You can also start a blog on how to mine crypto from apps because there are a lot of apps available today for mining crypto.