Here’s how The Blackbird Effect describes themselves:  ” The Blackbird Effect= Pan Morigan & John Cabán, performing bent blues, green jazz & other Americanismos with roots to the sky!”  “Americanismos,” that’s some fancy talk for a banjo and guitar duet.  Strange but alluring. You can check them out tonight at 7 p.m at Gateway City Arts,  92-114 Race St., Holyoke. 413-650-2670.