The problem with the Blue Card, from my [non-German/EU citizen] perspective, is that it's a work visa, which means you must have a job offer, and one that pays at least 37K EUR annually, to get the visa. Likewise if you lose that job, you've got just a short time to find another one or get out. The Blue Card is not a visa for those expecting to work at a German startup and compensated mostly with equity. In other words, not a visa for most founders.