Do a search at the Beginning and General Topics forums. This has been discussed numerous times here.
In a nutshell, Butterfield, Sonny Terry, Toots Thielemans (chromatic), Stevie Wonder (chromatic), and myself, are predominantly or exclusively NOT Tongue Blockers are all have excellent tone. Most modern pros use both.
Tongue Blocking will NOT give you a better tone if you are doing the Whistle Method (Pucker, Vertical Slot, Corner Blocking, etc.) properly. Most players do not by default drop their jaw low enough to create a "Vertical Slot" for single notes. It is the expansion of the oral cavity that creates a fuller tone and not the fact that your tongue is or isn't on the harmonica. The harmonica does not know the difference.
Tongue Blocking, by the nature of the technique, helps to open your oral cavity to create a better tone. But, when done correctly, the Whistle Method wil create the same or better tone when you focus on the "Vertical Slot" and expand your oral cavity.
It's much, much easier for the average person to learn to bend and bend in general, with the Whistle Method vs. Tongue Blocking.
Remember, it's all about opening up the oral cavity as much as you can to create the biggest tone possible. Dropping your jaw and getting he harmonica further into your mouth also forces you to use correct breathing vs. merely sucking and blowing with your mouth.