Swamping the table with huge hordes of models is nothing new to 40K, it’s just how 6th edition has made it possible for every army to do it- to have their cake and eat it also so to speak.
What is the definition of a blob in 40K?
Let’s start with that.
A blob is a huge unit of models, or multiple units of lots of models that work together as a single massive unit, cheap in points, very effective for their numbers, and made fearless/stubborn from a special ability or IC that has joined and hides in the unit.
By themselves the single model isn’t that impressive- imagine a chaos cultist or an imperial guard trooper, BUT taken in big blocs, say around 50-100 and now you have a mass that can absorb lots of damage and throw out lots of shooting and assault in quantity which becomes its own quality.
Allies has made the blob problem even greater since now every army (except the space bugs) can take cheap chaos cultists, zombies, or IG as blob fodder and still have all their other power armored and elite core army units.
That said, what really scares me about a blob is its powerful ability to deny parts of the table, especially the mid field in the game- advance those 100+ models to the center, restricting your opponents movement ad laugh as they can’t kill enough of the models in 5-7 game turns.
Or even better, bring the blob to one of your flanks, denying that part of the table from your opponent so they have to go to the other side and deal with the elite parts of your army- jacked up terminators, heavy weapon teams, etc.
So how are we going to deal with blobs?
Well, the first question to ask is do you have something in your list that can remove dozens of models a turn? A trio of punisher tanks? Mass templates, especially cover save ignoring ones? Do you have the mobility to get them into position against the blob, while your opponent is shooting at them to try and stop them?
Assault is another option, notice “option” but generally not the best one, as often the most you can hope for it to tie the blob up since they will be fearless and can’t be cut down, and then you still have the same issue of the blob denying the table.
Do you have the mobility, or mobile elements of your list that can fly over the blob, thus denying its space denial on the table?
Of the three options #1 is the best, followed by #3.
Blob + allies + flyers = 6th edition “competitive” play so having a counter is important as you will come up against it sooner or later…