Question: query regarding yellowing of blackgram plants. 
Answer: --first of all you have to do the oozs test for bacteria. 
procedure:
 you have to cut a piece of affected plant part and put into a glass of water, kept it for 20-30 minutes. if the water become yellowish than please apply copper oxychloride 2gram per liter of water in the root zone and also spray the recommended doses of copper oxychloride in the leaf portion also. if the water remain clean than please apply  glyoxilidin or carboxin 2gram per liter of water in the root zone and also spray in the leaf portion also 3 times at 4 days interval in the evening time in the absence of rainfall.