U.S. retail sales of consumer electronics on Black Friday dropped 5.6 percent this year, a "steeper than anticipated decline" that could signal trouble for the rest of the holiday season, according to market research firm The NPD Group.

The drop followed last year's loss of nearly 4 percent, the firm said. Black Friday tech shoppers this year went for PCs, Android tablets, and TVs, which together accounted for 58 percent of all sales dollars, up from 51 percent in 2009.

"This slow start is merely a continuation of the challenges seen in the consumer electronics business throughout 2012," Stephen Baker, vice president of industry analysis at NPD, said in a statement. "In an unbalanced market, where just a few categories deliver significant dollars, and even fewer offer any growth, the ability to deliver positive results will remain difficult for companies exposed to the entire consumer electronics marketplace."

Unit volume sales of flat panel TVs grew 4 percent overall, though revenue dropped 6 percent as a result of the average selling price (ASP) falling from $367 last year to $333 this year. Forty percent of all flat-panel volume came from 32-inch TVs, but with an all-time low ASP of just $194, these sales "dragged overall revenue down," NPD said.

On a brighter note, flat panels 50 inches and above saw a unit volume increase of 65 percent. The "star of the segment," however, was 60-inch and above flat-panel TVs, whose sales increased 10 times over 2010, accounting for 6 percent of all TV unit volume compared to less than 1 percent two years ago.

It was also good news for Android tablets, which saw a 177 percent increase in unit volume, and a 91 percent increase in revenue, even though ASP dropped to $151 from $219 last year.

Meanwhile, notebook units fell 10 percent, with sales of Apple notebooks flat and Windows notebooks down 10 percent. Windows 8 devices, with an ASP of $368, represented 89 percent of notebook sales.