Why are biofuels usually blended with standard jet fuel when used?
The issue with the biofuels is the low aromatic content, which changes the behavior of the fuel tank sealants. That's why blends are needed for the time being.


  (...) very low aromatic contents may cause seals and sealants to shrink and
  harden leading to acute or chronic failure. Unfortunately, most of the material qualification tests are more concerned
  with excessive swelling than shrinkage and there is little guidance offered as to an acceptable level of shrinkage or
  other changes in physical properties related to low aromatic content (NASA).





  At this stage,
  approval has been given for airlines to operate flights
  using the BtL and HEFA processes up to a limit of 50%
  biofuel and 50% conventional fuel. This is a precautionary
  measure enabling the industry to start using biofuels
  while additional assessments are undertaken on the
  need to maintain required levels of aromatic content
  in fuels. Aromatics are hydrocarbons found naturally in
  fossil-based fuels and are a necessary component for
  conventional jet fuels, forming up to 25% of the volume.
  They are not found in biomass-sourced fuels and this
  restricts the use of jet biofuels to 50% blends in order
  to guarantee aromatic content. The restriction allows
  time for an assessment of whether a synthetic aromatic
  source needs to be developed (Air Transport Action Group).





  Biofuels and FT fuels are very poor in aromatic content. This causes problems with
  seal shrinking and thus aromatics must be added to drop-in fuels in flight tests such
  that a concentration of 8% is achieved (European Commission).





  (...) aromatic hydrocarbons are
  more aggressive and cause more swell in seal materials than do paraffinic hydrocarbons
  
  (...)
  
  After the second cycle [of a jet fuel analog], the volume swell dropped
  from +22% to +13% as would be expected from the
  lower aromatic content in the fuel. However, this second cycle also caused the compression set to increase dramatically from 3% to 43%. Upon drying, the
  material again returned to its original volume, and
  compression set improved slightly. The reason for this
  significant increase in compression set is not obvious,
  but it is concerning (SAE International).




Trivia: there's been at least one demonstration flight on a jet that used 100% biofuel.