Which was the last Boeing crash that didn't send its black boxes back to the US?
Please refer to the following website for more details

https://slate.com/technology/2019/03/ethiopian-airlines-737-max-black-box-sent-to-paris-why.html

The International Civil Aviation Organization, which sets the standards for aircraft accident investigations, says that the country where an incident occurs is in charge of the investigation

If the concerned country has the tool to analyse the black boxes, they are analyzed locally and if necessary with the help of the aircraft manufacturer, therefore

When the concerned country has the knowledge to do the analysis there is no point to send the boxes to the manufacturer country.  What is particular to the Ethiopian case is that even though not having the tools to do the Analysis they preferred the help of a third party, rather than sending the boxes to the US, but this is beyond the scope of the question.

So the Ethiopian government asked Germany to analyze the black boxes, but according to CBS, the Germans also lacked the necessary software, consequently Ethiopia asked the help of France, thus the boxes have been sent to the Bureau of Enquiry and Analysis, the BEA.