After installing DupeBlocker my test coverage dropped below 75%, how can I resolve?
Your low test coverage is due to how Salesforce detects code dependencies and their related unit tests. You'll need to go to Setup->Develop->Apex Test Execution and choose Select Tests, then change the drop down to "[My Namespace]", select all classes and click run to validate your full test coverage and verify you're above 75%. Alternatively, you could just run your unit tests as you are now and verify that every class and trigger is at or above 75% coverage. You should be able to deploy your code to production without issue as long as you've verified the coverage of the individual classes and triggers. If you run into an issue with a DupeBlocker unit test failing during deployment there are a couple of different methods you could utilize to get around this. The primary method is to use the Force.com Migration Tool and add a runAllTests="false" parameter to your deployment target.
You might also try creating a changeset for your code and deploying that changeset to your production org.
Using the Force.com Migration Tool is Salesforce's recommended method for situations like this. The tool and documentation can be found here:
NOTE: Starting in DupeBlocker 3 developers can also supply their own test data to be used by DupeBlocker. This will allow supplying field values that conform to any customization's in the user org. More information on creating custom unit test data can be found here.