For example, a Colt blue label came from the factory in early 1994 with a flash supressor, but no bayonet lug or collapsable stock.  Is it legal now to add the collapsable and lug?  I thought I knew the answer to this question, but the more I read on this site, the more doubts I have.

As long as it had enough evil features to qualify as a grandfathered AW, there's no legal problem at the federal level with adding more features to it later.

Individual states may have prohibitions in effect other than those of the 94 AW ban, however, so know your local laws, too.

I think that if it's legally "evil" then you can make it as "evil" as you'd like.  I believe the removal of the bayonet lug by Colt was in response to legislation in CA in the late 80's.  Since even post-ban AR-15's are no longer welcome in CA, the whole "Blue Label" point is moot.

Someone please correct me if I'm wrong.

The "Blue Label" designation is the Colt transition models that come in a box with a blue label.  They are known as distinct guns as Colt volentarily built them with sear blocks and no bayonett lug or collapsable stock.  This was done for "politically correct" reasons starting about 1987-89 (correct me here on date).  Yet these guns sold before 9-94 are all prebans for sure - Colt did not sell stripped lowers.

I like them of all the prebans most.  Disagree? Well most are full A2 configuration, most are small hole lowers (no annoying adapter pin requited) and the sear block gives the lower strength.  Only downside is many custom match triggers don't fit the receiver with the block.  

Those that pay more for guns without the sear block I think are foolish.  You CAN remove these Colt searblocks - but why?  Either 1) you have a registed DIAS or 2) you make your own DIAS or 3) you will install a match trigger.  If the second reason is correct, have fun at Club Fed.  

To me, a match trigger in a preban AR is kinda silly.  There are many cheap post ban receivers out there that will make great match guns at a fraction of the price.