Black lacquer is a refined lacquer with color added, it has similar urushiol and water content to red lacquer if it is a top black, if it is a middle black the content may be slightly lower. Top and middle lacquers are just as they sound, top are of sufficient quality to be used as a top, exposed coating, middle lacquers are of a quality that they require a top layer of lacquer to cover them.

The schedule for tapping lacquer is very detailed, running from June – November and involves starting on the sunny side of the tree close to the ground and slowly moving the score marks around and up the trunk. The final stages of harvesting involve chopping the tree down and splitting the limbs to scrape out the lacquer. This final procedure gives an especially strong adhesive lacquer called seshime. The quality and amount of sap obtained is dependent on the person doing the tapping. The new tree will sprout from the stump.
The schedule below is from a manual on tapping.
Middle of June to middle of July. Hatsuurushi Tree is slightly scored, flow is minimal.
Middle of July to third week of August. Moriurushi. This is when the best quality sap is collected.
Middle of August to third week of September. 　Osourushi 　and/or urame, tome, and edaurushi.

Middle black lacquer is used over both sabi, (jinoko or tonoko/lacquer mix) and mugiurushi, (flour/lacquer mix) as a layer to give you a smooth finish. Without a middle layer of lacquer it is very difficult to get a smooth enough finish to successfully apply a top layer of lacquer and then metal. Middle lacquer is not as hard as top lacquer.