Wow, Blu is a beautiful horse!! I love his head...he's got such soft looking friendly eyes...
Back to the topic, I really do think his mane is a bit too long to plait 'professionally'. You could do it, but I think the plaits would turn out too big and lumpy.
Plaiting does require a shorter mane for that 'perfect' look (usually). My guess would be around a 3-4 in mane (that's my guess).
Maybe you could try and braid down the middle instead?
Painting his mane to match your tack would look cute!