This is an issue occasionally for some cultivars of green beans, including Blue Lake.  When you see flat and stringy beans, it is a reversion.  Reversions are not unusual for green bean cultivars so seed producers have things they can do during the seed production process to help eliminate off-types or reversions and reduce this issue for the end consumer.  High quality seed producers do this process regularly.  Low quality producers may only do it occasionally or when they start to hear complaints.

There is very little that can be done at this point to solve this issue.  The best way to avoid this is to buy seed from reputable company that have good seed maintenance programs.  Price is often (although not always) an indicator of quality.  This is because it is more costly to produce high quality seed that would not have issues with reversion.  

Overall the companies listed at this link are reputable.  There are other companies that are also reputable not on this list and no endorsement or criticism is implied to those listed or not listed here.  Hopefully it can give you some additional options to source your seed in the future.
https://yardandgarden.extension.iastate.edu/faq/what-are-some-good-sources-flower-and-vegetable-seeds

--Aaron