Depends on what is being done-simply diluting hemolymph in a low pH buffer (pH~ 5.0 or anticoagulant buffer) or something like ascorbic acid  will inhibit melanization.  

Another option is to use Phenylthiourea (PTU, 1 mM) is a good inhibitor of phenoloxidase and is often used in insect plasma to prevent melanization. Example reference is PMCID: PMC3656302

You can also try with the 2-Hydroxy-2,4,6-cycloheptatrien-1-one (Tropolone) 

Hi please try the solution by Azambuja et al. 1991 as

:0.01M ethylenediamine tetraacetic

acid, 0.1M glucose, 0.062M NaCl, 0.026M

citric acid, pH = 4.6

good luck,

Depends what do you needs, for examplo I need isolate SERPIN and I used antigualant buffer. NaCl (62mM), Glucose (100mM) EDTA (10mM) SOdium citrate (30mM) and Citri acid (26mM) pH 6,5 Reference Vargas-Albores et al 1993