To understand blocking, one has to understand the concept of optical density (OD). In an interference filter, OD is generated by reflecting unwanted light back instead of letting it go through the filter.
Proc. SPIE 10505, High Speed Biomedical Imaging and Spectroscopy III: Toward Big Data Instrumentation and Management, 105050G (2018); http://dx.doi.org/10.1117/12.2290773 -- WINNER! Pi Photonics Best Paper Award
We design and produce filters for many different applications. Each application has specific, often complex and demanding requirements. The level of performance attained in an optical system depends upon the integration of the filter design with the performance of other system components.