Blue screen when starting BitrixVM in VirtualBox can be caused by problems with virtual machine settings (memory, processors, network), outdated version of VirtualBox, corrupted BitrixVM files, errors in Windows system files or drivers. Check VirtualBox settings, update it, redownload BitrixVM, run sfc /scannow, update drivers and temporarily disable antivirus. Also, examine the Windows event log for errors. Make sure virtualization is enabled in BIOS. Try creating another virtual machine to check the basic operation of VirtualBox.