Why is my stock flavorless?
Couple notes or suggestions;


Added in fat?  Typically for ramen adding in a copious amount of finely chopped pork fat is a must.  Usually boil a slab on top of bones for 4 hours and then chop and add.  Serve with a side of lipitor.
Where is the savory flavour or unami?  You need to either cook some veggies/aromatics to get the Maillard reaction or cheat and add some MSG and/or a unami super boost (mushroom, marmite, anchoivies or other).  Lots of techniques to get savory flavour; which I think is what you may be missing.  soy sauce, or miso are also classic; pending what your end ramen is going to be.


Overall the technique is good; just missing the flavoring step; the base broth itself is not going to have a lot of flavour since you are not roasting; along with blanching step.

Blanching for a clear broth does remove a significant amount of flavour; but is 
more classical ramen approach.

Another trick you can use to extract more flavour is add a acid early to process; 1TBsp to 1/3cup or so of apple cider vinegar.  My goto when making a thai sour chicken soup or some other white broth soup that will be loaded with lime and cilantro later.  Drastic flavour change; so be-careful and test first.