There are different shades of blue. The word xanh by itself is a corruption of 青 (thanh in Sino-Vietnamese) and carries the general meaning of blue-green (since the two colours were historically seen as just different shades of the same).
Xanh da trời = sky blue
Xanh biển/dương = sea/navy blue (dương is more of sea/ocean blue)
Xanh lá (cây) = green
Then you also have xanh lục and xanh lam which can be used to distinguish between a generic green and blue.
This speaker often pronounces 'Một' as 'Một n'
Also, anh and ang do sound similar if you're not used to listening for the difference. nh is articulated further forward in the mouth than ang though, so it has a bit of a higher pitch. It is somewhere in between n and ng. nh is similar to the spanish ñ but just a little bit further back.
English speakers don't differentiate between the sound in king from the sound in kong. The sound he's making here sounds like the ng from king.
The pronunciation things mentioned above are something I only really notice when I hear Northern speakers. When I hear southern speakers I don't notice it much. Feel free to pronounce it either way. I speak with a southern accent but people in the north can understand me just fine