The "black screen" is the Grub (Grand Unified Boot Loader) menu. It can be
modified from within Ubuntu to lengthen the timeout before it goes
automatically to the default operating system. You'll have to get advice
from others about the details as I am a Kubuntu user, but it involves
editing /etc/default/grub, as root, to increase the grub timeout. You can
also change the default. After any changes it is necessary to run
update-grub to make the changes permanent.

There is no easier way to switch from Ubuntu to Windows than by rebooting,
unless you run one operating system in a virtual machine within the other.

If this is all Greek to you please ask for clarification!