Comment by inigyou
20 hours ago
They're completely different niches. Blow molding helps you make a million of a thing, 3D printing helps you make a million different things. There's great for low volume items without significant strength requirements. That includes prototypes but also anything niche - like parts for blow molding machines.
Another comment mentioned you can CNC a blow mold. That's completely true and I'm not versed in it enough to know when you'd prefer subtractive or additive rapid prototyping. A 3D printer is a bit more flexible in the shapes it can make but it can only make them out of plastic, which is a huge tradeoff.