It is plausible blinds will reduce heat loss but only through the Velux windows, not the rest of the extension.
Assume (say) a 3m x 4m extension with walls 2.4m high abutting the existing house. There would be (3+4+4)x2.4 = 26sq m of exposed wall, windows, doors. The roof, depending on pitch will add at least 12 sq m.
In total there is ~40 sq m of exposed building. 6 standard size Velux windows cover ~3 sq m.
You may do well to consider the quality of existing wall and roof insulation - if a modern extension there may be little that can be done, but older extensions may have been less well built.
Just fixing the Velux windows is most unlikely to make a great difference to your heating costs overall.
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Velux blackout blinds 
Velux blackout blinds 