I have obtained, as a Dutch citizen, a B1/B2 visa for the USA recently and plan to travel for business to Iraq in the coming weeks. Would the visit invalidate my 10 years valid B1/B2?
You would no longer be eligible for an ESTA but I know that is not the question.
Your visa is still valid just be prepared for extra questioning, be honest about why you went and take evidence of the business you went to do and why you went. They may not even ask you unless they spot the Iraq visa in your passport, even then they might not be bothered. Just do not lie if they ask you if you've visited Iraq.
It would only affect an ESTA because it would invalidate some of the questions you get asked when applying. The questions on a B1/B2 visa don't ask if you have visited any of the countries that stop you getting an ESTA.