Stocks fell hard today, adding to a rough end to the week on Wall Street. Following a quick jump after the Federal Reserve announced its interest-rate decision on Wednesday, indexes have now given up four percentage points in the past two days.

In today's session, the Dow Jones Industrial Average (DJINDICES:^DJI) lost 371 points, or 2.1%, and the S&P 500 (SNPINDEX:^GSPC) fell by 36 points, or 1.8%. Both indexes are in the red year to date, with just seven full trading days left in 2015.

^DJI data by YCharts.

The trend of weakening crude oil prices continued: Prices fell by 1% today, with per-barrel cost sitting at seven-year lows of below $40. Meanwhile, individual stocks on the move today included car retailer CarMax (NYSE:KMX), and tech giant BlackBerry (NYSE:BB), which both posted their third-quarter earnings results before the opening bell.

CarMax's declining customer traffic
CarMax was the worst performer on the S&P 500 today, falling 7% after posting softer-than-expected third-quarter earnings results. Sales of used automobiles, which make up most of its business, declined 1%, marking a significant turnaround from the prior quarter's 5% gain. The drop was driven by a decrease in customer traffic, management explained. "We had a challenging sales quarter," CEO Tom Folliard said in a press release.

Image source: CarMax. 

Net income also posted a surprising decline, falling 1%, to $128 million. Yet stock buybacks ensured that EPS in spite of top-line issues: CarMax reported profit of $0.63 per share, or 5% above the prior year's $0.60 per-share haul. Still, consensus estimates were targeting more-robust earnings growth, to $0.68 per share.

In a conference call with investors, CarMax executives provided details on the used car sales decline, attributing it to factors including aggressive promotions by new-car dealerships, a drop in the supply of older cars, and rising wholesale prices for SUVs and trucks. These issues held growth back in the quarter, although CarMax still managed to sell 154,000 cars overall, a Q3 record for the retailer. Yet the declining store traffic, and uncertainty around when that trend will turn back again toward steady growth, had investors selling the stock in heavy trading today.

BlackBerry's surging software sales
BlackBerry shares jumped 11% higher on surprisingly strong third-quarter earnings. The mobile tech-and-solutions specialist posted a double-digit improvement in adjusted revenue, thanks to a near 200% sales bounce out of its software-and-services division. The overall $557 million revenue figure trounced Wall Street's expectations of $489 million. BlackBerry beat consensus estimates on Q3 profit, as well. Its $0.03 per-share loss was better than the $0.14 per-share loss that analysts were bracing for.

The BlackBerry PRIV. Image source: BlackBerry.

Meanwhile, PRIV, the company's newest mobile device, has had a good reception since its launch last month, executives said. "We're seeing positive feedback and good demand thus far," CEO John Chen said in a conference call with investors.

That news, along with improving profitability and software sales trends, has the company believing that it can add to the strong momentum it posted this quarter. "To sustain our current direction, we are stepping up investments to drive continued software growth and the additional PRIV launches" to different carriers around the world, Chen said. Executives predicted accelerating sales growth across BlackBerry's software, hardware, and messaging businesses in the fourth quarter.