It's generally necessary to apply for visas in your place of residence, not in your country of nationality. The US recommends this, but does not require it. (There is an exception for US visa applications whereby people who have overstayed a US visa in the past have to apply in their country of citizenship. I suppose this does not apply to you.)
The process is outlined on the website of the US embassy and consulates in Germany. Applications in Germany are processed by a third-party service provider at http://ustraveldocs.com/de/.
The recommendation to apply in your place of residence may be found on the State Department's website, for example on its page concerning visitor visas:
You should schedule an appointment for your visa interview at the U.S. Embassy or Consulate in the country where you live. You may schedule your interview at another U.S. Embassy or Consulate, but be aware that it may be more difficult to qualify for a visa outside of the country where you live.
One reason for this recommendation is that the consulate will want to verify your ties to your place of residence. It is much more difficult for a consular officer in India to verify your employment, your blue card, your residence, and so on, than it is for a consular officer in Germany to do so.