Same as in Chinese, the concept of 'blue' as a colour exists in Welsh, but it's more extensive - and covers a swathe of colours from deep greens, sea greens and misty greys.
The word 'glas' derives from the ancient Celtic word for 'woad' (blue dye the Celts painted themselves with) = 'glastum' in Gaulish, a word which survived the extinction of Gaulish as it was appropriated into Latin. Since 'glastum' is a deep blue dye, the idea that the Welsh had no concept of 'blue' via their word glas is wrong, but woad could also produce a range of washed-out light blueish greys and blue-greens if used in weak concentrations, or with different treatments. (The Chinese word for blue has a similar origin - and connects to the colour effects of woad as a dye too.)
QI is sort-of correct. Traditionally, Welsh divided the spectrum differently from how English speakers (and most modern Welsh speakers) do. In old-fashioned/poetic usage, glas can cover green, blue, grey, silver and so on. Natural colours like the sea or plants. The Irish word glas went the other direction and now mostly means "green". https://en.wikipedia.org/wiki/Distinction_of_blue_and_green_in_various_languages#Celtic
It would be understood as a direct equivalent to English "blue". But you do get to see the old way in place names and words like glaswellt (glas + gwellt, "green grass").
Makes glaswellt gwyrdd sound quite funny really! I think gwellt without glas in front has come to mean "straw" though.