The music of Chicago — gritty urban blues — is famously about hard times, heartache and struggle. But practitioners of the genre may boast a refreshed if wary air of accomplishment this week, upon favorite son Barack Obama’s ascension to Democratic presidential candidate. At least, that’s my thesis, which I’ll test by listening close to some of the 90 performances at the City-sponsored, free downtown 25th annual Chicago Blues Festival June 5 – 9 — and probably a slew of after-fest blues in neighborhood taps scattered around the toddlin’ town.