Blaziken has a base speed of 80. That's pretty poor even in NU, and Speed Boost Blaziken is an Uber tier Pokemon.
So lets assume all your Pokemon & your opponent's have 31 IVs in speed.
Blaziken will be at 196 with no EVs. After 1 Speed Boost (x1.5), its speed will be 294.
However, with full speed EVs, its speed will be at 259. After 1 Speed Boost (x1.5), its speed will be at 388.5. That's enough to outspeed:
- Timid Gengar (350)
- Modest M-Gengar (359)
- Modest Mewtwo (359)
- Timid Darkrai (383)
- Timid Greninja (377)
- Timid Shaymin-Sky (388; speed tie)
- Jolly M-Lucario (355)
So yea. Speed EVs are definitely worthwhile on Blaziken, because it got base 70 defences which means your chances of getting another speed boost without attacking are slim.