Germany is part of the Schengen treaty for free travel among its member states (see http://www.auswaertiges-amt.de/DE/EinreiseUndAufenthalt/03_Visabestimmungen/Visabestimmungen.html); beyond this, Germany does not require visa for all Western and many Eastern European countries.
Moreover, Germany has extensive visa waiver agreements with many other countries, including Argentina, Brazil, Australia, Canada, Chile, Hong Kong (SAR passports), Israel, Japan, South Korea, Malaysia, Mexico, New Zealand, Singapore, Taiwan, and the United States, among many others. For the other countries (e.g., China, India), visa processing is usually smooth and with a predictable (positive) outcome.
For visa requirements in full detail see the following link: http://www.auswaertiges-amt.de/DE/EinreiseUndAufenthalt/03_Visabestimmungen/StaatenlisteVisumpflicht.html.
Lessons learned from other conferences, e.g., ACM MobiHoc 2016, tell that little visa issues arise for travelers attending a conference in Germany. There are no “blacklisted” countries. The only problems reported in the past were arising from people working or studying in the US and were not able to travel because they might run into problems returning to the US.