Block number in the Tens indicates major rebuilds of the airplane. In the ones it indicates changes, but not major rebuilds.
Close, but first digit is the production block/lot and not a "rebuild" indicator. Block or Lot numbers numbers are used to differentiate the Block/Lots as they typically but not always) correspond to the year(s) the Block/Lot were built. Upgrades such as MLU/CCIP* in the case of the F-16 would indicate a "rebuild" or upgrade to improve capability and standardize configurations.
*MLU = Mid Life Upgrade (participating foreign operators)
CCIP = Common Configuration Implementation Program (USAF)
The F-15, F-16 and F/A-18 each had corresponding upgrades to increase capability and standardize configuration. In the past, these would have been called CILOP (conversion in lieu of procurement...ie F-4B to F-4N or F-4J to F-4S) but Congress is very hard on new type/model/series so aircraft have kept their initial type/model/series designation like F/A-18A, EA-6B or E-2C but have been significantly upgraded through Engineering Change Proposals (ECP) that usually have an associated acronym like ICAP III or a popular name like Hawkeye 2000.
CJ is block 50/52. C denotes single seat (c model) J is basically production run.
Again, close, but the CJ model is something special and was developed to perform the Wild Weasel mission replacing the very specialized F-4G model Phantoms that trace their lineage to the definitive Vietnam Wild Weasel, the F-105G. The CJ has the Harm Targeting System (HTS) integrated so the CJ can perform the SEAD mission effectively and that is the primary distinction of the CJ over other F-16C models. Back to thread, the Navy flies the traditional Iron Hand mission dating from Vietnam but never introduced specialized units like the Wild Weasels even though certain A-6 Intruders had considerable capability (Shrike + Standard ARM). Unlike the F-16, all F/A-18 series can carry HARM as could later A-7E and A-6E aircraft.