Here’s the rule:
The term “blackout period” means, in connection with an individual account plan, any period for which any ability of participants or beneficiaries under the plan, which is otherwise available under the terms of such plan, to direct or diversify assets credited to their accounts, to obtain loans from the plan, or to obtain distributions from the plan is temporarily suspended, limited, or restricted, if such suspension, limitation, or restriction is for any period of more than three consecutive business days.
29 C.F.R. § 2520.101-3(d)(1)(i) https://www.ecfr.gov/current/title-29/part-2520/section-2520.101-3#p-2520.101-3(d)(1)(i).
As Luke Bailey points out:
Even if there is no disruption to an individual’s power to direct investment (because the plan does not provide such a power until after the recordkeeping change is completed), a blackout might result if there is a practical inability to get a loan or distribution.