One clarification and one comment.
Firstly, your posture and RoE are NOT taken into account in the Boxes, neither Blockade nor Shipping. So if your frigates are in one of the Blockade Boxes searching for fugitive Blockade-Runners, leave them on the Defensive posture Blue/x(what-ever). This will maintain their cohesion the longest. If you are searching for them in open-waters, then you will have to be on Offensive/Attack (O/O) -- no, kids, those are not breastesses

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The chance of actually catching a Blockade-Runner if both your frigates and the BR have high cohesion, is rather small. Once the BR's cohesion starts to drop through Time-At-See and the affects of weather their cohesion will drop and they will become much more likely to be caught and the chances of them getting smacked pretty hard is greater because they will not be able to withdraw from a battle as easily.
The the major tip to both sides is to keep your shipping in top shape.
On A Side Note
Ship-repairs cost an equal percent of their damage to their purchasing cost. So if a brig that costs $12k and 10WS and has 10 HitPoints takes 5 hits, goes back to harbor, is repaired, goes back to blockade running, takes another 5 hits and is repaired again, the owner will have paid the same amount for repairs as if the brig were actually sunk and subsequently replaced. The only thing that you do not have to pay for during repairs is fresh crew-members. These are a free-bee for what ever reason.
You do not have to leave your ships in harbor long enough to get them completely repaired and once the crew has recovered their cohesion they will perform equally as well on the damaged ship as on an undamaged ship; they just won't have the same number of guns and hits making them weaker in combat, but equally as good on evading combat, which is what you want.
I generally leave my brigs at the docks only long enough for the crew to recover and then sail them back out so that they start bringing in money and WS again as quickly as possible. Remember, as long as they are not in the Blockade-Boxes, they are not earning their keep