1. The microcode does not reside in the boot block. It is a hidden part of BIOS, and you want to load the latest 3.88 BIOS as the only practical way to get the latest microcode. Rarely HP has released a microcode as a separate updater..... not ever for the Z420. If you have an original version of the motherboard installed currently then the latest microcode likely will not help you much if at all, but other BIOS improvements are worth having, in my mind. So, get that now, and install it.
2. The boot block date is the easiest way to see if you have the original or the newer revision 1 version of the motherboard. Only if you have the latest of the two boot block dates will you be able to run the later v2 versions of the processors on that motherboard, and you also need to choose from a processor that HP specifically has approved for that revision 1 later motherboard. There may be some very rare exceptions, but don't count on it and don't blame HP if you end up wasting funds. They don't actually "support" processor upgrades from what you had on the original motherboard that came with your workstation. Having said that many of us follow these rules and succeed with HP-approved processor upgrades.
3. You cannot update the boot block date..... it is locked to the motherboard, and even if you get the latest microcode by updating to the latest BIOS you will not be able to run the newer HP-approved v2 processors if you have the older boot block date.
Hope this helps......
