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It's all relative. Rent can range anywhere between €500 per month or more. It's reasonable to expect an average of €1000 - €1500 per month. While those prices may sound expensive to some, you tend to get more for what you pay equivalently in the most expensive cities in the U.S. (e.g. San Francisco, New York, etc.).
This, again, is relative. You may want to explore Penha Franca, Madre Deus, or Alfama if you want to live close to the city centre. Otherwise, living a bit outside of Lisbon (e.g. Cascais, Oeiras, Almada) may be a better option.
Lisbon has a growing Black ex-pat community that includes Black Americans, Africans, and Afro-Latinos. Many African shops can be found in Rossio (a historic city square). There is also a large presence of African residents in areas like Chelas and Cova da Moura.
This depends on the type of passport you hold. European residents/citizens do not need to apply for a visa if they hold a Schengen passport. Americans can receive a tourist visa on arrival once they land in Portugal. However, to extend your visa term (i.e. beyond 90 days) you will need to go through a formal visa application process.
Lisbon has a growing tech startup scene with several opportunities to pitch for funding or invest in a company. Many ex-pats also come to Lisbon to purchase real estate that they may rent using services like Airbnb or Booking.com.