QB Issue Resolution:

It marks the date line between today and tomorrow. You can’t remove it.

Resolution for Issue 'What is the solid blue line in my register? How do I remove it?' available: Yes (Solved).
Source: Intuit Community forum.

What is the solid blue line in my register? How do I remove it?: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

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