My big question is “How much does Blue hydrogen cost above natural gas?”
A: I do not know the answer but you are asking the right question.
Energy is a total commodity, only the lowest cost player (or fuel) will survive. This is why coal displaced wood for heating homes, then natural gas displaced coal. Natural gas is now displacing oil for both heating and electricity generation. Now solar and wind are starting to displace natural gas for heating (solar PV plus a heat pump is half the cost to heat your home than natural gas) and for transport (solar PV plus a Tesla costs 2c per mile vs. gasoline at 10c per mile). It is game over for fossil fuels. Unfortunately it will take 20 years. But the outcome is now inevitable.
I doubt that even green hydrogen will be low enough cost to be able to compete with renewables and heat pumps for heating or renewables and electric vehicles for transport.
The only way to make green hydrogen today is to use solar or wind electricity to split water molecules into hydrogen and oxygen. This is fundamentally less efficient (i.e., more expensive) that just using the renewable electricity directly. There may be some niche applications, such as using excess solar electricity (that would otherwise be wasted) during sunny days to make hydrogen and then burning the hydrogen at night to run a power plant. But other than temporary energy storage, I just do not see hydrogen being competitive with solar, heat pumps and EVs.