I'm designing a PCB and I was looking at the schematic of the blue pill STM32 board to integrate it, and found that there are two crystals on the schematic? Why is this? Also, there is only one on the board?
It is common for MCUs and developement boards to have a high speed main clock crystal and a low speed crystal for real-time clock feature where it can keep time with only backup battery power. STM32 calls these HSE and LSE.
The STM32 does not need either crystal to function, it's just that the HSE is useful and might even be required for some tasks. The LSE is omitted because most people do not need it, but can add it if they want it.
If you intend to start using STM32 MCUs, please understand that the Blue Pill is a terrible example, it does many things that should be avoided or not done.
You should refer to manufacturer documentation and examples how to use STM32 properly.