Blocking Response Page seems to be blocked

When a request is blocked by ASM, that happens before the connection is ever proxied to the back end server. You can configure a custom blocking response in ASM so that, when ASM does block a request, the client is redirected to a different location, such as your custom PHP form. See this article here: https://support.f5.com/kb/en-us/products/big-ip_asm/manuals/product/asm-implementations-11-5-0/25.html

Although it's for 11.5, you can check the equivalent on later releases.