I am also a great admirer of the electric blue of the Dark Star. I think you could keep it the size you want, with only some lost of the bloom. Pruning it you would probably find a lost of bloom every other year as it grew back from the pruning. It is a relatively short lived plant for me, but with better care yours should last longer. Al

You'd be better off putting it in another spot with more room to let it have the space it prefers. Three foot tall by three foot deep for this cultivar isn't realistic long term if you also want it to bloom. I have seem Ceanothus 'Carmel Creeper' kept hedged to this size successfully, but leaf size and flower color aren't a good match.

If you are new to Ceanothus, my advice... plant them in FALL!