We also had this problem for about one year now, struggling with 
different methods self-made or commercial to block endog. biotin. 

Sometime the liver is "clean", sometimes not, same with kidney and 
stomach. As it seems to be unpredictable whether the tissue under 
investigation needs long-time blocking or not, we finally decided to 
switch to another detections system: 
using a secondary antibody coupled with FITC and then a third anti-FITC 
coupled with HRP and then DAB. As we did not want to look at an actual 
FITC-signal, you do not have to work in the dark. We tested the system 
regarding sensitivity etc and compared to our original 
streptavidin-biotin system. 

There was no difference, price is the same, you can buy it as a kit 
(pre-diluted, multilink), it takes only 10 minutes incubation time for 
the second and the third layer, you save time because you do not have to 
block endogenous biotin and you are sure in the end that you have a true 
positive signal. I can highly recommend it. I do not get any money from 
our supplier, and may be there are other companies selling the same kit. 
We use the NBA-bulk kit from Zytomed.

hope this helps,

Dr. Heike Grabsch,
Dept of Pathology
University of Duesseldorf
Germany