Blackbox driver: Failed step "No user with Test User name is registered with the driver."

Thanks for the quick reply. I'll have a look at the Doobie code.

Just for the curious here's an excerpt from behat.local.yml.example

# Local configuration.
default:
  context:
    parameters:
      drupal_users:
        drupal:
          'drupal'
        site user:
          'site user pass here'
        git user:
          'git user pass here'
        git vetted user:
          'git vetted user pass here'
        docs manager:
          'docs moderator pass here'
        admin test:
          'admin test pass here'
        site maintainer:
          'site maintainer pass here'
      #git usernames don't always match the drupal username
      git_users:
        gituser:
          'gituser pass here'
        gitvetteduser:
          'gitvetteduser pass here'

And here's the step definition from \doobie\features\bootstrap\FeatureContext.php

/**
   * Authenticates a user with password from configuration.
   *
   * @Given /^I am logged in as the "([^"]*)"$/
   */
  public function iAmLoggedInAs($username) {
    $password = $this->fetchPassword('drupal', $username);
    $this->iAmLoggedInAsWithThePassword($username, $password);
  }

JFTR

I think I will solve the problem by using meta steps as described at http://knplabs.pl/blog/behat-like-a-boss-meta-steps

EDIT 1:
Following the instructions from the above URL I came up with the following feature using steps provided by the Mink extension.
(This is not yet a real meta step):

Feature: User login to Drupal
  In order test features only available to registered users
  As a user
  I need to be able to log in to the Drupal site

  Scenario: Log in to the Drupal site as user Dagobert Duck
    Given I am on the homepage
    When I fill in the following:
        | Benutzername | Dagobert |
        | Passwort | test |
    And press "Log in"
    Then I should see the text "Prof. Dagobert Duck" in the "header"

Added links to already researched resources.