This should be impossible.
The GPL states that anyone who modifies GPL licenced code (in this case the Linux kernel) is required to make available the sources used to compile it. Blackview agreed to the terms of the GPL when they started using the Linux kernel in their products.
When you're saying it's impossible to provide the kernel source, you're saying that Blackview is perfectly fine with being in breach of license. If this is Blackview's official policy in regards to licensed software, it's really quite shocking.