Your brain thinks of these two words completely differently and it's only through conscious effort that you think of them together. They are different words even though they sound and are spelled the same, regardless of the space.
A better example I think is "bear feet" vs "bare feet"
"Blackbird" refers to several species of actual bird, not just the plane. "The black bird ate seeds" and "The blackbird ate seeds" are both reasonable sentences, and they do potentially sound different.
I'm not convinced. It takes effort for me to break apart blackbird into two separate words in my head, as they are so commonly found together. When speaking "black bird" I would insert a long pause between the two and emphasise the "b" on bird to show that I'm not talking about a "blackbird".
The black bird ate seeds.
The blackbird flew at mach 3.
Your brain thinks of these two words completely differently and it's only through conscious effort that you think of them together. They are different words even though they sound and are spelled the same, regardless of the space.
A better example I think is "bear feet" vs "bare feet"