Can Blake2 be a replacement for double SHA-256?
BLAKE2 can be used as a drop-in replacement for SHA-256d, yes. It is not vulnerable to the length extension attack. Whether or not you should use it in cryptocurrency is a more complex question, but it suffices to say that it will not be any less secure than using SHA-256d for your purposes.
Regarding the speed comparison, this will be more complex to answer as you'd need to specify if you want to encourage CPU mining, GPU mining, or ASIC mining for the cryptocurrency. The raw speed of the hash on a modern CPU will also vary, but BLAKE2 will be faster than SHA-256d. Exactly how much faster depends on the CPU's capabilities, such as SIMD features supported.