If you distribute a blackout notice to participants and do not meet the original end date on the blackout notice, is there a formal procedure to follow when you extend the time?

You're supposed to send out another notice. The regulations say, "the administrator shall furnish all affected participants and beneficiaries an updated notice explaining the reasons for the change and identifying all material
changes in the information contained in the prior notice."

"The updated notice must be provided as soon as reasonably possible, unless such notice in advance of termination of the blackout period is impracticable. In this regard, the Department reiterates that to the extent that an administrator has the ability to furnish notice to some participants and beneficiaries earlier than other participants and beneficiaries, which may be the case where electronic disclosure is available, the administrator has an obligation to provide such notice, even though providing advance notice to other participants and beneficiaries (e.g., by mail) may be impracticable."