To create a service without an appointment date/time, create a package which does not contain services/sessions (i.e. an empty package).
Click Add package from the Fast Action Button in the bottom right corner:
Set the package name to the name of your service, e.g. "Blood Work", add the fee and a description (optional) describing the service offered.
When you click Save, you'll see the following prompt:
Click Continue since you do not intend to offer a service with a required date/time.
If a client books this package from your Bookings Page, they will not be asked to book a session, only prompted to pay (if a fee and payment settings are set up).