He moved to Somerset for the 2001 season (with a nothing batting record in either game-form, which has improved dramatically since).
Yardy has simply been fortunate with circumstance. His first 2 games were on 2 of the slowest, most turning pitches you'll ever see in England. Then he played 2 games against Ind and Aus where the pitches were tricky and so was the smallish run-chase, so he got played-out. I don't doubt he'll be back, though - that's just the nature of English ODI selections. I thought we'd seen the last of James Kirtley in 2003... but then he was recalled for 2003\04... ahead of first Gough then Johnson, both of whom had infinately better cases. Once Yardy came-up against a batting-line-up who went after him on a half-decent wicket, he went for plenty.
As for Blackwell, his situation isn't incredibly dissimilar. He was perfectly OK in the VBS of 2002\03 but his best 2 performances came on the two worst batting surfaces (SCG opening game, Adelaide in the Anderson-10-12-1 match) and he somehow got 2 wickets in 3 overs in another game. Then his next serious ODI series where he bowled at all was in Pakistan where he had 2 reasonable games followed by 2 poor ones, then got another woeful batting surface in the last game and took 10-29-3. And in India the pitches were mostly bowler-friendly too.
Giles' record could IMO quite easily be worse, too - ER of 4.45-an-over-ish, and has not played that much (53 games).