With Ubuntu 18.04 LTS or Ubuntu 18.10 and full disk encryption enabled, after I install the nVidia drivers I also receive the blank screen though I can type the passphrase in and it will continue to boot successfully. What seems odd based on some past comments is adding the "nomodeset" parameter for me presents the splash screen as expected even with the nVidia drivers installed. I've tested this with both the 390 and 415 drivers.
With Ubuntu 18.04 LTS or Ubuntu 18.10 and full disk encryption enabled, after I install the nVidia drivers I also receive the blank screen though I can type the passphrase in and it will continue to boot successfully. What seems odd based on some past comments is adding the "nomodeset" parameter for me presents the splash screen as expected even with the nVidia drivers installed. I've tested this with both the 390 and 415 drivers.