CS Brigs in the Blockade Boxes are considered Blockade Runners and bring some money and occasionally a WSU per turn. When the Brigs return to port they also are given a bonus of money and/or WSU, but there are dangers involved. Entering and leaving the Blockade Boxes, when Blockade Flotillas and/or other patrolling combat ships are present, greatly increases the chances of the Blockade Runners being caught(1) and possible forced into battle, especially when they've been at sea for an extended period of time and their cohesion is lowered. Therefore they should always be set to evade combat, and many say to PP (Passive Posture), bc it will enhance their chance of escape, but it will also increase the damage they take if they are caught, so you'll have to decide where you want to take your chances.
Blockade Runners require extensive management, especially if you use a lot of them. The last time I played the Confederacy, I filled the Gulf Blockade Box with all the Runners I could build. I used the Gulf Blockade Box only, bc from an income point of view, which box you use makes no difference, and unless you have bad whether or your Runner has lost a lot of cohesion while in the Box, you can generally get them from the Box into harbor (Mobile) in one turn. That means two turns longer in the Box collecting money while cycling them if they can make the trip in one turn. The Atlantic Box always takes two turns to get to harbor, regardless of which harbor you were to use.
So every single turn you have to look at ALL of you Runners to check their supply status, bc when the run out of supplies at sea they start to take hits and lose huge amounts of cohesion, which means it take even longer to get back to harbor, which means even more hits and cohesion loss, and plus repairing hits is costly and time consuming--basically the Runner might as well have stayed in port--, so it's something you rand to avoid like the pest.
(1) As the Union player, if you are trying to catch Raiders in the Shipping Lanes Box (I don't recall it working as will will the Blockade Boxes), build some small Hunter-Killer™ squadrons of a couple Sailing Frigates (ships), a few Steam Frigates, maybe 4 or so, preferably with a naval leader, and cycle them into and out of the Shipping Lanes. They must be in OP (Offensive Posture) while outside in the Boxes, for them to attack encountered ships, but in the Boxes and while entering and exiting the boxes, other rules apply, and they can be in DP as well, which lessens the Wear-n-Tear™ while in, entering or exiting the Boxes.
Cycling your Hunter-Killer™ squadrons in and out of the Shipping Lanes greatly increases the chances of them finding a Raider, but it also greatly increase Wear-n-Tear™, which means they quickly lose cohesion and may even take hits from bad weather and just general Wear-n-Tear™ while sailing in OP, which greatly lessens the chances of them finding a Raider. I only ever cycle them into the Shipping Lanes twice at the most, before they must return to harbor to recover.
You can also just leave them in the Shipping Lanes in DP if want to try your luck at passively patrolling, but from my experience, the chances of finding a Raider passively is pretty low.
Raiders in the Shipping Lanes can greatly lessen the money and WSU the Union gets for their Transports in the Shipping Lanes.