I always use blues on yellow or lower morale players cuz you need 2 to make him superb...anyhow I dont buy blues but still my overall blue reservs have slow but steady increase...soon there will be 200 of them in bank so I dont really care is better this way atleast I can use them would be lame having 1000 of them in bank with no use
Is good this way

Also players might get tired of winning or clean sheets and they need some motivation...thats why morale drops
But seriously if you spec matches and win alot you should have enaugh blues to keep team in very good/superb morale all the time