Okay, your talking specifically about Blue-Water Blockading as opposed to Brown-Water Blockading; I was thinking of the latter.
I've never actually looked into the Blockade Values of each type of ship since as the Union I'd learned that Blockade Flotillas (Blockade Squadrons) have the highest ROI (Return On Investment).
Union ships have the following Blockade Values:
Steam Frigate: 4
Armored Frigate: 4
Blockade Ship: 10 (can only be purchased within a Blockade Squadron)
Additionally, ships/boats that cannot actually reach the Shipping-/Blockade Boxes also have the following Blockade Values:
French and English ships have the same values except that their Brigs have a Blockade Value of 3 instead of 2. They also have a class of ship they simply call 'Warships' which appear to be Ships-of-the-Line. These have Blockade Value 3 and are something like very large steam frigates. The US laid down a number of these type of ships, but only a few were actually built and still on hand during the CW. Their value as fighting ships had long since wained but their size made them useful as receiving and auxiliary ships.
As far as I know the Blockade Value plays no role in Brown-Water Blockading since I've only ever seen the actual number of ships/boats being counted and not their Blockade Value. I've used Blockade Squadrons for Brown-Water Blockading, but not seen that they actually function any differently than the same number of gunboats.
If the Blockade Value of a ships is what is counted for determining the Blue-Water Blockade Value vs the CSA's trade, then the Blockade Ship is by far the best ship for this as it's Blockade Value us twice that if a Sail-Frigate. The Sail-Frigate does have the best detection value though which makes is idea for raiding or defending against Raiders. I like to put a number of frigates into the shipping fleets to help them avoid and defend against Raiders.