The Mortgage Bankers Assn. said 6.19% of a sample of 9.2 million home loans surveyed were at least 30 days past due. The figures from this year's first quarter were the highest that the association has found since it started keeping track of mortgage delinquencies in 1953. The previous record of 5.83% was set in the third quarter of 1984. Industry experts blamed the rise in delinquencies on the nation's sluggish economy, persistent regional pockets of high unemployment and slow appreciation in home values.