To unambiguously count the number of blobs requires a precise rule, but you could change the rule and the number of blobs would be different according to the new rule. That much is indeed not nonsense.

But when we talk about "the branch containing this instance of you", we are talking about something that definitely exists, and whose existence is not just the result of how you define things. That's why it's nonsense to treat it as if its very existence results solely from an arbitrary division of the universal wavefunction.