Khloe Kardashian -- Lamar Odom Poisoned My Brother

Khloe Kardashian clearly blames Lamar Odom for contributing to her brother's drug problems -- and says she feels guilty for not being able to protect Rob from L.O.'s influences.

As we previously reported, Rob had gotten deep into sizzurp and weed -- and his family was pushing for him to go to rehab.

Now, Khloe is talking about Rob's issues -- and without specifically mentioning the drugs -- says Rob's problems stem from the time he lived with Lamar Odom, who has serious drug problems of his own.

"Rob has said, 'Being in that house it was so dark and it made me do dark things,'" Khloe said on "Keeping Up with the Kardashians."

Khloe added ... "Only Rob and Lamar know what happened in the house and know what's going on."