As TimG mentioned you need to provide assurance to your customer. If I was your customer I would not be happy with the current situation, even with the metal detector downstream of the equipment. The problem is the metal detector only detects what it can detect and it really is not a fail safe solution...it only DETECTS.
You need to do a root cause analysis and determine why you are breaking blades as such a frequent occurrence AND can you somehow reduce the occurrence of blade breakage. Additionally, you need to ensure your SOP around a broken blade is very robust.
Don't get lulled into being "safe" just because you have a metal detector.
Now, if you have done all this and really there is no way to reduce blade breakage further, your metal detector verification & validation is very robust, and your break procedure is very robust then you need to have a conversation with the customer to provide them assurance because of XYZ the blade can in fact be broken, but your product is still completely safe and here is why.