I've been browsing the web for a while, but can't find an answer. I normally don't bother with prologues but for this story I'm writing, I need to give some kind of context before I start it. The prologue is around 150 words and I don't want to put anything in that I'll put in the story. Should I just alter it to be a blurb or is that an okay length?
I believe you can do anything you want, it's your book.
If the story needs a prologue that's only 150 words long, then do it. It would be far worse practice to pad the prologue out with unnecessarily information because of some arbitrary rule enforcer who tells you it should be longer.
As DoubleU has stated, you don't have to title it 'Prolouge', it's far more interesting to come up with something unique that arouses the reader's curiosity.
For example, Half Bad by Sally Green (exceptional book) has two "prologues", but they read more like very short chapters with headings that continue through the novel. One is titled 'The Trick' and is only 154 words long and the other 'The Cage' which is only 54 words long.
Another example is He Said She Said by Erin Kelly (another great book). This also has two "prologues". One explains the phases of an eclipse and is only 122 words long. The second is a more traditional prologue, a clip of the story later on, at 494 words. Erin doesn't title hers at all, they are just centred on the page. She then uses images of eclipse phases to divide the book. It's unique and interesting and tied into the characters who chase total eclipses.
Nip into your local book store and take a peek at these two novels, they may inspire your short prologue.
In short, do whatever you like. Just make it intriguing for the reader to want to continue turning pages.
Most prologues, and epilogues, I'm used to seeing are on the order of half the size of the average chapter for the rest of the work so that is the benchmark I always think of with prologues and epilogues.
That being said I've read prologues that only a sentence or two and epilogues that are longer than any two chapters in the rest of the piece put together. So while there seems to be a "standard" both prologues and epilogues are fragments that can be as long or as short as they need to be to convey the information that you think needs to be given to your audience.