Well, this January 31, 2018 article provides a few things. Of course, I found it by accident and missed it when I was deliberately searching.
The Spanish IT company Indra will be working with Zero 2 Infinity. The Bloostar is designed to lift 75 kg to low earth orbit. They also said that Zero 2 Infinity has some balloon missions to take care of, and Bloostar will be put on the "backburner while revenue-generating services take priority."
That article referenced a March 14, 2017 article on the launch of a prototype firing a single engine, and the article mentions that Bloostar uses liquid oxygen and methane. As of that time they had not yet crossed the Karman line. Bloostar is also designed to be recoverable, and they recovered the prototype. Ultra Magic, a hot air balloon manufacturer, is an investor in Bloostar. As of March 14, 2017, they had planned to make their first commercial launch in 2019. So it probably won't happen this year, because space is ALWAYS harder than people think it is, even when they say yes, they know space is hard.
According to this March 28, 2018 article Zero 2 Infinity is getting 3D-printed combustion chambers for its Teide 1 engines from the Foundation for Aerospace Development (FADA). An Andalusion non-profit, curiously enough. The combustion chamber is for their Teide 1 engines, named after in inactive volcano in Spain's Canary Island archipelago. Six Teide 1 engines will be on the second stage and a single Teide 1 on the third stage. Each capable of 2 kilonewtons of thrust. So, tiny. Six larger Teide 2 engines are on the first stage, each providing 15 kilonewtons. Still small.
Working with CATEC, the company plans to apply artificial intelligence (AI) and neural networks to engineer more effectively cooled thrust chambers.
"Traditional rockets have had straight cooling channels because that's all that could be manufactured," Jose Mariano Lopez-Urdiales, founder and CEO of Zero 2 Infinity, said in a March 22 statement. “When you put a flashlight in your ear, you see a wonderful tree-like structure of blood vessels. We don’t have straight rows of blood vessels in our ears. 3D printing and AI now allow rockets to evolve, like nature.”
So that's more than I knew before. Some interesting stuff there.