Doing blackburns to mimic your overhead press pattern is a good way to encourage good shoulder mobility, rear delt activation, a neutral lower spine, and an extended thoracic spine without any of the vertical stress forces on the vertebrae or shoulder capsule. As a bonus, the rest of the posterior chain gets hit hard too.
Rather than using the floor or an unsecured bench, which limits how much tension you can create through the entire posterior chain, secure your heels in the glute-ham raise or horizontal back extension machine. It makes everything between the back of the knees and the rear delts work very hard, both isometrically and isotonically.
The most important part of this lift is the hand and arm position. Use a thumbs-up position for some external rotation, and try your best to mimic the conventional overhead press angles while being horizontal.
That means the head travels through the "window" created by the arms at full extension, and the arms stay in line with the shoulder joint and spine. It also means keeping the elbows tucked toward the floor as much as possible at the bottom of the lift.
The 5-pound plates shown in the video should be adequate for just about anyone. Focusing on a slow tempo and maintaining positioning will prove challenging enough, and even a pound too heavy can ruin your patterning.
This exercise is great way to help train overhead range with a different reference point, allowing gravity to create a new force angle. It's excellent for healthy lifters, and often a safe alternative for lifters who feel pain during pressing movements.